Menzies Campbell yesterday publicly rebuked a Liberal Democrat activist for starting campaigning for a by-election before the former incumbent was even buried.
The funeral of Eric Forth, Conservative MP for Bromley and Chislehurst, is due to take place today after the 61-year-old died from cancer last week.
But on Monday, Lib Dem London campaign chairman Peter Dollimore sent out an email to activists providing information on how to begin campaigning for the by-election, expected to take place in June or July.
Mr Forth, a member of parliament since 1983, had a 13,000 majority at the last general election, but the Lib Dems, who came a close third behind Labour, believe they have a good chance of making headway in the area.
However, speaking at a lunch in Westminster, Sir Menzies made clear his disapproval of Mr Dollimore's actions - by convention, parties should not start campaigning for a by-election until after the late MP's funeral.
"Eric Forth was a friend of mine. Eric Forth was born within a mile of me. He went to school about half a mile away," the Lib Dem leader said.
"I deprecate any effort to begin electioneering under any circumstances in any by-election until the writ has been moved by the party with responsibility for doing so. If that were done I disapprove and I will make sure my disapproval is known."
At the same event, Sir Menzies was forced to admit once again the trouble he had with prime minister's questions (PMQs), saying it was a "rather different experience from other parliamentary occasions we have to face".
But he insisted: "I remember Paddy Ashdown found PMQs a rather different experience and David Steel and Roy Jenkins. It is a work in progress. It is a challenge and I am determined to meet the challenge.
"If PMQs was the only political test in this country William Hague would still be leader of the Conservative party."