Former Chief Secretary to the Treasury Liam Byrne has said he regrets writing a "flippant" note to his successor informing him "there's no money left".
The letter from the Birmingham and Hodge Hill MP was opened by Liberal Democrat David Laws, who has since stood down from his new role.
It read: "Dear Chief Secretary, I'm afraid there is no money. Kind regards - and good luck! Liam."
Mr Byrne told the BBC's Politics Show it was a "foolish letter to write".
When the letter dated 6 April - the day the general election was called - was made public earlier this month by Mr Laws, Mr Byrne said he had just been using a phrase that all chief secretaries got used to.
In an interview with the BBC's Politics Show for the West Midlands on Sunday, he conceded it made Labour's job of challenging the coalition government's policy on cutting the deficit more difficult.
He said: "It was a foolish letter to write.
"The truth is there is dark humour between politicians at Westminster - even when you're knocking things out of each other.
"I broke the golden rule which is to not write down anything that you are not happy seeing in public and I am sorry that it's made our job arguing against Tory plans harder when I think those plans are wrong.
"I'm sorry it made me look flippant about deficit reduction when anyone who has worked with me at the Treasury knows that I took deficit reduction seriously.
"I'm sorry it's made our job arguing against the Tories' plans for fast cuts harder."
Mr Laws, who became Chief Secretary to the Treasury in the new coalition government, said at the time he thought the letter would contain advice from his predecessor.
"Unfortunately, when I opened it, it was a one-sentence letter which simply said 'Dear Chief Secretary, I'm afraid to tell you there's no money left,' which was honest but slightly less helpful advice than I had been expecting," he said.
But Mr Laws, MP for Yeovil, resigned his post on Saturday after admitting he claimed £40,000 in expenses to pay rent to his partner, following allegations in the Daily Telegraph newspaper.
He said he wanted to keep his relationship with James Lundie private.