The deputy leader of the London Assembly Conservatives has insulted Liberal Democrat deputy leader Simon Hughes on Twitter.
James Cleverly wrote: "We may be coalition partners but it doesn't stop me thinking Simon Hughes is a dick."
Caroline Pidgeon, Liberal Democrat leader in the London Assembly, called the tweet "juvenile and abusive".
A Conservative spokesman declined to make a comment and Mr Hughes could not be contacted.
The tweet was made as the coalition government marked its 100th day in office.
Mr Hughes, MP for the south London constituency of Bermondsey and Old Southwark since 1983, is Nick Clegg's deputy leader but does not have a government role.
Mr Cleverly was responding to Mr Hughes' suggestion that backbench MPs should have a veto on the coalition government's policies.
In fuller comments, made on his blog, Mr Cleverly called Mr Hughes a "fool" and said the idea was "bone-headed".
He added: "Simon Hughes clearly feels that he is the 'real' voice of the Lib Dems.
Responding, Ms Pidgeon AM said: "These juvenile and abusive insults about a well-respected politician simply because James disagrees with him are demeaning.
"They reflect far more on James himself."