The Liberal Democrats have been hit by the defection to the Conservatives of one of their MEPs.
Sajjad Karim, who represents the North West Region, said the Lib Dems "have lost their way".
Monday's announcement comes as the party seeks to regain its footing after the sudden resignation as leader of Sir Menzies Campbell.
And the debate between the two contenders to succeed him, Nick Clegg and Chris Huhne, has also become increasingly fractious.
Praising David Cameron's leadership, Karim said the Conservative leader's recent speech on immigration had prompted the decision to defect.
"Two years ago David Cameron described himself as a liberal Conservative and that's very much how I see myself," Karim said.
"But I needed to be convinced that he could change his party and now I am.
"David Cameron's recent speech on immigration was incredibly impressive – sensible, rational and positive.
"It persuaded me that he is a leader who has changed his party and will change this country.
"I'm afraid that the Liberal Democrats have lost their way and are no longer a serious force in politics.
"I'm here to serve the people of the North West and I think I can only do that now as a Conservative."
Cameron praised Karim as "a hard-working MEP who has so much to offer".
And Timothy Kirkhope, Conservative leader in the European Parliament, said: "I know that he will contribute much to our work as the largest British delegation of the parliament and as we pursue our policies for a better Europe."
But a Lib Dem spokesman suggested the decision had been made because Karim was in second place in the party's list for the next European parliamentary elections, making it less likely he would retain his seat.
26 November 2007