Advising Mayor Boris Johnson must be infuriating. His team had pulled together a sensible if not earth-shatteringly revelatory report on arts and the recession in a bid to garner support for funding the arts in tough times.

They had brought on board Kevin Spacey, director of the Old Vic, Simon Robey, UK head of Morgan Stanley and chairman of the Royal Opera House, and Mark Jones, director of the V&A, to speak at a launch attended by many of the arts world great and good. And Boris torpedoed it.

With the impeccable magpie tendency of a man who picks up ideas rather than concentrates on finessing the fine print, he suggested that London’s museums and galleries might implement voluntary charges as exist in New York -  which he visited last week to promote tourism.

It was a strange intervention, not least as anyone who visits London’s museums cannot but have seen the collecting boxes beseeching donations. Moreover, as a cursory attempt to test the New York system would show, the “voluntary” or “suggested” donation at the Metropolitan Museum of Art or the American Museum of Natural History is anything but. The moral arm-twisting is heavy, the deterrent factor to the poor considerable.

Boris did not, for the record, advocate abolishing free admission. But museum leaders immediately pointed out the danger of confusing a clear-cut, popular and successful policy, one which has already claimed the scalp of  Hugo Swire when, as shadow culture secretary, he dared suggest charging should be up to the museums themselves.

Most importantly, the idea was a serious diversion from the matter in hand. Around 70 per cent of private investment in the arts in Britain goes to the arts in London but most organisations have seen a fall of support in the last year. It is crucial to maintaining a thriving cultural sector that that investment is encouraged to return.

Simon Robey admitted that outrage at the excesses of the financial industry in the last year or so had shamed many potential supporters into shunning fund-raising corporate hospitality events. Yet what was really needed was for bankers and hedge fund managers to come out en masse and cough up their bonuses in a good cause.

As Spacey argued, the arts is a business and one in which the UK excels. It warrants the serious investment that any serious revenue-generating industry expects. At least some of those behind Boris evidently recognise that. They can see the threat to thousands of jobs if the delicate mixed economy eco-system of arts funding fails. But does he? It is sometimes hard to tell for the quips.