But one thing certainly has changed in my three decades as an aisle-squatter: the role of the critic has been enlarged and expanded. When I began, one was largely a straightforward reviewer. Now, as the theatre industry has grown, one is expected to take on a number of other roles – profile-writer, preview-lister, obituarist and commentator on the politics and economics of theatre. Everyone knows the old joke about the critic who went straight home after the theatre he was attending burned down. Asked the next day by the news editor why he didn't at least ring the desk to inform them, he suavely announced: “I am your dramatic critic, not your newshound.” Needless to say, such Addison De Wittery would be impossible today.

I welcome the expansion of the critic's role: indeed one of the first things I did on joining The Guardian was to bombard my poor arts editor with think-pieces on such topics – this was 1971 – such as “Where are the women dramatists?” But the idea of the critic as multi-purpose commentator has lately come under attack. In particular Adrian Noble's precipitate departure from the Royal Shakespeare Company and Trevor Nunn's decision not to extend his contract at the National Theatre by another five years were both blamed on the press. Earlier this year – immediately after Noble's announcement of his resignation – I took part in a radio discussion with Jude Kelly, the retiring director of the West Yorkshire Playhouse. She accused me, along with other critics, of hounding Noble and Nunn and of exercising, in Kipling's old phrase, the harlot-like prerogative of “power without responsibility.”

Is it true? Has the same kind of media mob-rule that partly prompted the resignations of Peter Mandelson (twice) and Stephen Byers, and has had Ken Livingstone in its sights, led to the departure of Adrian Noble and Trevor Nunn? Only they can say; but I suspect the charge is nonsense. Far from abusing their power, I think critics have, if anything, been rather lax and kid-gloved in holding the directors of the RSC and the National to account.

Take the case of Adrian Noble. He first announced his plans for a radical shake-up of the RSC – including a restructured company, an exodus from the Barbican and the creation of a “Shakespeare village” in Stratford – in May 2001. The initial reaction was cautiously questioning rather than downright hostile. The only critic vehemently opposed from the very start was John Peter in The Sunday Times. My own attitude changed only in September 2001 when, as a result of several letters and phone-calls I received, I began to understand the mounting fury within the RSC and the Midlands at large at Noble's proposed changes. I wrote a highly specific piece in The Guardian urging Noble to re-think his plans. Other critics started to take a similar line.

Patience snapped

The case of Trevor Nunn is stranger and more personal. I have not the remotest idea why he chose not to renew his contract at the National Theatre for another five years. But the idea that he was the victim of sustained critical abuse is absolute rubbish. For the most part his stewardship of the National has been greeted with a fawning approval bordering on total sycophancy. Only two critics, myself and Nicholas de Jongh in The Standard, persistently questioned his policy at the National Theatre, and even then I think we were both fair-minded enough to praise individual productions to the skies.

What intrigued me was the number of times I was asked: “What have you got against Trevor?” or “Why are you running a vendetta against him?” Clearly I had failed to get my point across. There was no hidden personal agenda. What I questioned was the cautious conservatism that characterised Nunn's approach to the National: in particular the heavy reliance on long, uninterrupted runs of commercial musicals – Oklahoma!, My Fair Lady and South Pacific – and the signal failure to expand and explore the European repertory. I have never denied Nunn's gifts as a director or the quality of individual productions: Troilus and Cressida, Summerfolk, The Merchant of Venice. My criticisms have been based on his lack of any idealistic vision of what a National Theatre could be.

Is this an abuse of my power as Guardian theatre critic? I see it more as a legitimate exercise of the authority temporarily invested in me by the paper. I don't deny that critics are lucky dogs. We have the space and freedom to say what we think. But it seems to me as fair for a theatre critic to hold the publicly-funded National Theatre to account as it is for a football writer to question the tactics and strategy of Sven-Goran Erkisson. And, while I got a certain amount of flak for my questions about Nunn, I also received some encouraging support: from key figures inside the National, from other theatre people nervous of going public and from readers all over the country.

The critic has to call it as he or she sees it, and that obligation is ever more important at a time when spin is as prevalent in the arts as it is in politics. Subsidised companies employ large publicity and marketing departments whose job it is to buff up and polish the corporate image. And the commercial theatre depends heavily on PR. One particular operator is a master, before any major production opens, of parcelling out interviews with the stars, the director, the designer in order not just to sell the show but to nullify the power of the critic. If you think I exaggerate, look back at the vast amount of hype – emanating from another PR company – before the opening of The Lion King in London. One journalist was despatched to New York to tell us that it was actually better than Hamlet. Supposed preview pieces turned into glowing reviews. By the time the show opened anyone who thought, as both I and Sheridan Morley in The Spectator did, that it was a fine spectacle but a hollow show were consigned to the margins.

Which brings us to the power of the critic. I wouldn't deny it exists; but that power is provisional, transient and heavily qualified by the comparable muscle of the PR and celebrity industries. It doesn't take a genius to work out that the London theatre is currently in the grip of a star-mania that virtually renders critical comment redundant. With black-market tickets allegedly changing hands for £500 to see Madonna in Up For Grabs, it doesn't matter a damn that John Gross loved it, John Peter was outraged, or that most of us thought it was the non-event of the year. Even more sinister is the growing tendency of newspapers, with the connivance of the PR industry, to insinuate “reporters” into the first preview to pre-empt the reviews. I even did it myself, although The Guardian actually paid for the ticket, with Jerry Hall's debut in The Graduate and have lived to regret it ever since.

Limping along

The power of the critic in Britain has always been healthily checked by one simple fact: the abundance of national newspapers. God forbid we should ever reach the condition of America – not just New York – where each city has one dominant paper that dictates what shows people see, what shops they patronise, what restaurants they eat in. But if there is danger in an excess of critical power, there is an equal danger in critical impotence. If critics can no longer steer people towards shows that are unfashionable, unhyped or simply under-budgeted, then the whole theatrical culture will eventually suffer.

But I am not without hope. For one thing, broadsheet papers like The Guardian and The Times are currently giving greater space and prominence to reviews by placing them in their front sections; and, even if this means a return to the life-shortening pressure of the overnight notice, it at least restores to the theatre its old-fashioned news value. In the end, however, the power of the critic can only be restored by one simple thing: by our all writing with whatever passion, authority and wit we can muster. Power, in short, is dependent on good prose: if theatre-people objected, as they constantly did, to Frank Rich's clout at the New York Times it wasn't simply because of the supremacy enjoyed by his newspaper but also because of the vivid muscularity of his writing. And I was very struck by a passage in the Tynan Diaries where, in 1975, he talked of going to see David Hare's Fanshen at the ICA. “This is the first native offshoot,” he wrote, “of the Brechtian tradition that seems to me to stand comparison with the parent tree. Despite excellent notices, the small theatre was only half full. If I were a critic today, I would guarantee to have it packed.”

Few of us now would dare to make such a proud vaunt. And if the power of the critic has gradually dwindled, it is for all the reasons I have mentioned: the sheer intensity of the puff and PR industry, the psychotic cult of celebrity and the general impatience of a culture where people want quick, encapsulated opinion rather than detailed argument. But it ill becomes people in the appraisal business to whine. Criticism will regain the ground it has lost only when, however much we are castigated by the theatre industry as devils, we recover our ability to write like angels.