James Naughtie, who presents Radio 4's Today programme, has signed a deal believed to be worth £310,000 to write an unauthorised account of the relationship between Tony Blair and Gordon Brown.

Details of the book were being kept secret last night, but it is understood that Mr Naughtie will examine the heart of the government by focusing on the relationship between the pair. He was not thought to have told either man about his deal last night.

Literary sources speculated yesterday that the publisher, Fourth Estate, had paid too much for a political book after the company took part in a closed auction with eight other publishers.

Usually only a former prime minister could generate enough interest to attract such a fee. However, Mo Mowlam was reported to have sold the rights to her autobiography for £350,000 in December.

To avoid accusations that Mr Naughtie might compromise his position as co-presenter of Today by writing the book, it will not be published until after the next election.

Yet BBC sources suggested last night that the deal might still put some noses out of joint. Alastair Campbell, the prime minister's press secretary, is known for his eagerness to express displeasure at Today's treatment of the government and would be swift to react, warned one insider.

Mr Naughtie's fellow Today presenter, John Humphrys, caused controversy last year with his book Devil's Advocate. Senior BBC executives were livid at his open criticism of the corporation's news organisation.

Sources close to the deal said it would generate interest for its novel take on politics rather than for scandal. A source said: "The publishers got very excited about the idea of a book about politics which won't just appeal to political people."

Mr Naughtie has known Mr Blair and Mr Brown for a number of years from his time working for the Scotsman newspaper. He used to share a flat with Mr Brown.

A source said he intended to cover the rivalries and politics of the Labour party through their personal relationship and hinted that there would be previously uncovered stories.