Ex-Labour minister Elliot Morley has admitted dishonestly claiming more than £30,000 in parliamentary expenses, the largest sum of any former MP.
He pleaded guilty to making excessive claims for mortgage costs between 2004 and 2007 and pocketing cash for another loan that had been paid off.
Morley stepped down as an MP at the last election, having represented Scunthorpe since 1987.
He is the fourth MP to appear in court over expenses fraud.
His barrister, James Sturman QC, told Mr Justice Saunders that his client accepted a jail sentence was now likely.
"We know it is not a question of if but how long," he said, while urging the judge to consider the former MP's "lifetime of public service".
Morley has been released on bail pending sentencing, not due to happen before 12 May. He left Southwark Crown Court without speaking to journalists.
As environment minister between 2003 and 2007, he is the most senior politician to plead guilty to fraud since the expenses scandal began in 2009.
When the allegations against him first surfaced, he claimed it had been a "genuine mistake" and would seek to prove that he was not guilty of deception.
But the Crown Prosecution Service said Morley's behaviour had been "blatantly dishonest".
"He has accepted that his repeated and fraudulent claims for one mortgage that had been paid off and his excessive claims for another were not legitimate in any way," a CPS spokesman said.
"Mr Morley had claimed he was unaware the mortgage had been paid off. For most of us, paying off the mortgage is a red letter day and members of the public have found it difficult to comprehend his explanation that he was not aware that had happened.
"The Parliamentary expenses system exists to assist the public's representatives in carrying out their duties, but Mr Morley used it to line his own pockets with just over £30,000 - more than an average household's annual income.
"Such behaviour is blatantly dishonest and cannot be excused."
Morley is the third former Labour MP to plead guilty to expenses fraud. David Chaytor and Eric Ilsley are now serving jail sentences.
A fourth Labour MP, Jim Devine, was the only one to stand trial but was jailed for 16 months after being found guilty last month.
A fifth parliamentarian, former Conservative peer Lord Taylor, was also found guilty of expenses fraud after a trial.
Morley, 58, pleaded guilty to two charges of false accounting. He admitted to receiving £15,200 more than he was entitled to in inflated mortgage claims and of submitting £16,800 in "phantom" mortgage claims after he had redeemed the existing loan on a property near Scunthorpe.
Morley wrongly filled out a total of 40 forms relating to mortgage payments.
Morley was barred from standing for Labour again in a general election in 2009 after which he decided to quit Parliament.