A Midland Liberal Democrat MP - who defected from Labour - announced yesterday he was quitting Charles Kennedy's party and applying to rejoin Tony Blair's.
Paul Marsden, the MP for Shrewsbury and Atcham, said he was worried that Labour MPs in marginal seats could lose to Tory opponents - if Labour voters switched to the Liberal Democrats.
Mr Marsden's move will be an embarrassment to Mr Kennedy, who on Tuesday welcomed the defection of Stephen Wilkinson, who had been the Labour Parliamentary candidate in Ribble Valley, to the Liberal Democrats.
Mr Marsden hit the headlines in December 2001 when he quit Labour, voicing opposition to the Afghanistan war and anti-terrorism legislation.
His career took a colourful turn in November 2003 when it emerged that he had posted poems on his internet site which referred to his infidelity. Last November, Mr Marsden vowed to continue working despite discovering he had a bone tumour on his right shoulder.