But no one is entitled to know how much Shawcross was paid for the temporary installation, even though the museum receives £15 million of taxpayers’ money each year. The trustees have twice turned down a request under the Freedom of Information Act to reveal the cost of housing the artwork.

Richard Thomas, the Information Commissioner and guardian of the new Act, also refused to publish the figure. It appears that the fee earned by Mr Shawcross, 28, son of the authors William Shawcross and Marina Warner, is secret.

The commissioner’s refusal to disclose the size of the payment will be challenged next month in a key test of the efficacy of Britain’s new Freedom of Information regime. The case will go before the Information Tribunal on December 16.

If the tribunal rules that the price paid by a public body to buy or display a work of art is secret, then confidence in the Act, which came into force in January, will evaporate.

The legislation is already in trouble. The Information Commission is struggling to cope with a huge backlog of appeals against refusals by Whitehall and other bodies to disclose information to the public.

The case about Continuum is shaping up as a David v Goliath battle. It is being brought by Matthew Davis, a freelance journalist from Brighton who lodged a series of requests for information about the prices paid by galleries and museums for contemporary works of art. Many of his requests have been refused on the grounds of commercial confidentiality.

Mr Davis intends to represent himself at the tribunal, the members of which are appointed by the Government. He is being opposed not just by the National Maritime Museum but also by the Department for Culture Media and Sport.

He said: “It’s hardly a fair fight. I’m an amateur pursuing this case in my spare time.”

The price of public art has suddenly become a hot topic. In recent weeks it has been revealed that hospitals spent £9 million over two years on artworks and the Tate broke government rules by spending £600,000 on a painting by one of its own trustees.

It emerged yesterday that a BBC press officer “invented” reasons to explain why the corporation spent £60,000 on commissioning a sculpture by Tracey Emin. The work was installed in Liverpool at a time when the BBC was supposedly cutting costs.

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