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Change UK MP Chuka Umunna says he has switched to the Liberal Democrats because he was "wrong" to think "millions of politically homeless people" wanted a new party.
The former Labour MP for Streatham told Today he had "massively underestimated just how difficult it is to set up a fully-fledged new party without an existing infrastructure".
He says there's only room for one centre right party in British politics.
Former Labour and Change UK MP Chuka Umunna has joined the Lib Dems.
The Streatham MP told the Timeshe had been "wrong" to think "millions of politically homeless people... wanted a new option on the ballot paper".
"I also think I vastly underestimated the importance of having an infrastructure and existing relationships with voters," he added.
He was one of six Change UK MPs to quit last week, after it gained only 3.4% of the vote in the European elections.
During the European Parliamentary elections campaign the main UK parties will be interviewed on the BBC News Channel.