Tory defector Quentin Davies has urged other Conservative MPs to "take the plunge" at the Labour Party conference.
Mr Davies left the Tories to join the Labour Party
He accused Tory leader David Cameron of having made a "Faustian pact with his own extremists" on Europe.
He said he was proud of his new party and won a standing ovation from many delegates but others appeared to find the speech amusing.
Deputy leader Harriet Harman welcomed other ex-Tories, including Northern Ireland Secretary Shaun Woodward.
Mr Davies praised Gordon Brown as a man of "sound judgement" and "great competence" who had dealt with a series of trials since becoming prime minister.
By contrast he accused Mr Cameron of saying and doing "such consistently foolish and superficial and transparently contradictory things" - on schools and tax and spending.
Many delegates gave him a standing ovation, but others remained seated - veteran left winger Dennis Skinner, laughed as Mr Davies told delegates they should be proud to be part of the Labour Party.
Former Tory MP for Grantham and Stamford Mr Davies defected to the Labour Party in June, the day before Mr Brown became prime minister, accusing the Tories of ceasing "collectively to believe in anything".
Mr Cameron thanked him for his past support and told him "We will watch your future career with interest".