The Conservative leader, Mr William Hague, threw down the gauntlet to his former party colleague, Mr Shaun Woodward, the MP who defected to Labour at the weekend, and challenged him to "do the honourable thing" and resign his seat and fight a byelection.
Members of Mr Woodward's former constituency association in Witney, West Oxfordshire met last night and called on him to resign from Parliament and stand in a by-election. Mr Hague was clearly angry when he insisted he would not base Conservative policies "on one individual holding a gun to our head".
Rejecting Mr Woodward's claim that the Tories had lurched to the right and become "extremist" under his leadership, Mr Hague said his party would continue to reflect and campaign on the mainstream views of British voters.
"If he wanted to act as a man of honour he should have a byelection.
"He should do the honourable thing and stand down and fight a by-election against a Conservative candidate," Mr Hague said.
Mr Woodward was completing his defecting by signing his official Labour Party membership form yesterday, declaring his decision to defect had thrown his entire career into jeopardy: "I have no idea what will happen now. There are no detailed plans and no deals that have been done.
"It was simply that I could not remain in William Hague's rightwing Conservative Party any longer.
"The party I joined of John Major and Chris Patten was a party that spoke of opportunity for all.
"It was at ease with itself and the party which valued health services, schools and argued about Britain being at the heart of Europe and one which wasn't at all xenophobic. The new Conservative party has in the last two-and-a-half years changed dramatically and I could not find myself staying inside a party that didn't care about society in which we live."
The Education Secretary, Mr David Blunkett, predicted other disenchanted Tories might follow Mr Woodward's example and defect to Labour. "Obviously it is perfectly possible now, with the disintegration of the Conservative Party taking place before our eyes, that there may well be more of those incidents.
"Some will want to cling in and try desperately to change what is happening to the Tory Party, others will think it is a hopeless cause, as Shaun Woodward clearly does, and will want to get out," he told BBC Radio 4.
But another former Tory MP, Mr Peter Temple-Morris, who left the party in November, 1997, and joined Labour in June last year, spoke of the difficulties involved in changing political allegiance.
Mr Temple-Morris said defection was not easy, "it's an agony one has to go through. Once one has done it, rather like swimming the Channel or whatever, one feels much better for it".
A Labour Party spokesman said later that Mr Woodward's parents, Dennis and Joan Woodward, both in their 80s, who had been "lifelong Conservatives", had joined New Labour.