Mr Whelan's move will appal Downing Street, which hoped that his departure would enable the Government to put behind it the faction-fighting that also claimed the resignations of two ministers, Peter Mandelson and Geoffrey Robinson.
Since announcing on Monday his intention to leave the Treasury, Mr Whelan has already received offers from three publishers to write a book - including one worth pounds 200,000.
"He has been offered enough money to pay off Peter Mandelson's debts," said one friend of Mr Whelan, who has strongly denied leaking details of the pounds 373,000 personal loan from Mr Robinson to Mr Mandelson.
Although the Cabinet Office may try to scupper his literary plans, Mr Whelan argues that he can avoid breaching Whitehall rules by not disclosing any Treasury secrets. However, he believes the rules would not stop him writing about the tensions inside the Labour Party, including the rift between Mr Brown and Mr Mandelson, which dates back to the latter's refusal to back Tony Blair for the party leadership when John Smith died in 1994.
Mr Whelan has decided not to seek a job in the City, but is attracted by the idea of becoming a media pundit or taking a job in football.
Mr Whelan came under pressure yesterday from both the Government and the Tories to leave his post immediately. Although Mr Whelan is in no hurry, Peter Kilfoyle, the Cabinet Office minister, said he would leave "sooner rather than later" and senior Whitehall sources expect him to depart "within days".Reuse content