Gordon Brown's latest book project is British through and through

His call for the Union flag to fly in every garden was ridiculed by his opponents.

But Gordon Brown remains determined to bang the drum of Britishness.

Breaking off from Labour's leadership turmoil, he announced plans to write a new book on the subject of being British.

The Prime Minister told the Edinburgh international book festival that his third book in four years will be out next spring.

Bill Campbell, of Mainstream Publishing in Edinburgh, of which Mr Brown used to be a director, said: 'It was an original idea from Gordon and there has been a substantial personal contribution from him.

'He has approached several archbishops and other religious leaders for their views, and Ian Rankin, JK Rowling and Sir George Martin are among the other contributors.

'The book is meant to reflect on the diversity of what it means to be British in the modern day.'

Gordon Brown is a prolific writer for such a senior political figure.

Last week saw the publication of a short story by the Prime Minister in which he revealed his childhood hero was Scott of the Antarctic - who led his men to an icy death following their failed mission to get to the South Pole.

He has already published two books - Britain's Everyday Heroes and Courage: Eight Portraits.

He discussed Britishness with author Ian Rankin at the weekend when he was a surprise guest at the Edinburgh book festival.

He described the rise of the Scottish National Party as a 'phase', and said Scotland was better off remaining part of the United Kingdom.

The Prime Minister added that historians believed only three per cent of Scots had English relatives in 1707, the year of the union between the two nations. Most Scots had nothing to do with those south of the border.

Mr Brown said: 'Today, it's an amazing fact that 50 per cent of Scots have close relatives in England, so it seems strange that people are talking of splitting up when there's such a level of connection at a human level, at a family level, with England.

'We share the same values about liberty, about democracy, about the need for social cohesion and for people to work together cooperatively. We share respect for similar institutions like the health service.'

Last night Scots nationalists scoffed at the book idea.

A spokesman for SNP First Minister Alex Salmond said: 'His Britishness campaign has always been about him trying to sell himself to people south of the Border, especially Middle England.

'What they want from the Prime Minister is a decisive Government dealing with the economic crisis, not a floppy identity campaign.'