Nick Clegg is "self-obsessed", "dishonest" and "a revolting character", a former aide to education secretary Michael Gove has said.
Speaking to The Sunday Times, Dominic Cummings said Clegg was "the worst kind" of politician who "only cares about his image".
Cummings, who resigned as Gove's special advisor last year, said he and his colleagues "thwarted Clegg as much as we could."
Clegg was described as "self-obsessed, sanctimonious and dishonest" by the former advisor
He told the paper: "He is self-obsessed, sanctimonious and so dishonest he finds the words truth and lies have ceased to have any objective meaning, and he treats taxpayers money with contempt.
"He won't do the hard work to get policy right – all he cares about is his image. He is a revolting character. And I say that after spending 15 years in Westminster.
"Whenever Clegg gave a speech, he'd demand we spent hundreds of millions of taxpayers' money for his latest absurd gimmick. We thwarted Clegg as much as we could."
The Huffington Post UK contacted the Lib Dems for comment but had received no response as this story went live.
Lib Dem life peer Olly Grender, who is a close advisor to Clegg, took to Twitter to respond to Cummings, who, she implied, still had wounded pride over a "humiliating climb down" over exam policy.
The tweet referred to the 2012 clash between Clegg and Gove when the Lib Dem leader hit out over Gove's plans to replace GCSEs, which the education secretary ultimately abandoned.
Speaking at the time, Clegg said: "This was self-evidently not policy that has either been discussed or agreed within the coalition Government.
"Mr Gove is entirely entitled to come up with proposals and then, if he wants to, we can then discuss them within the government."