Former Conservative MP Sarah Wollaston has joined the Liberal Democrats, saying it was the best way for her to fight for the UK to remain in the EU.
Ms Wollaston was one of three Tory MPs to quit the party in February and join what would become Change UK, but she left that newly-formed party in June.
She follows ex-Change UK and Labour MP Chuka Umunna in joining the Lib Dems - which now has 14 MPs.
Lib Dem leader Jo Swinson said Ms Wollaston would bring "real expertise".
The Totnes MP quit the Conservatives in February over the party's "disastrous handling of Brexit".
She told BBC Radio 4's Today programme that she had now joined the Liberal Democrats because of their work in "making the unequivocal case for us to remain at the heart of Europe and tackle no deal, which would be a disaster".
It is a "time of national crisis" and "people wanted to see a single unified force making the case against no-deal", she said.
But Ms Wollaston backed Lib Dem leader Jo Swinson in rejecting Labour's proposal for a caretaker government. She said their plan to stop the UK leaving the EU without a deal was not "realistic".
Instead, she called for a "trusted figure who commands cross-party support", who was not a party leader, to take temporary control instead.
Mr Umunna told BBC Newsnight that Ms Wollaston's decision to join the Lib Dems was "massive" and underlined that it was the "biggest and strongest Remain party in this country".
When asked if the Lib Dems expected any more defections, he said there were "lots of conversations going on and there are lots of people on the train but only they ultimately know when they will arrive at the destination".
It is the latest boost for Lib Dem leader Ms Swinson - who was elected in July - following the party's success in the Brecon and Radnorshire by-election on 1 August.
Ms Wollaston, who was first elected in Totnes in 2010, left the Tories alongside Heidi Allen and Anna Soubry in February to join an independent group.
When asked if her newest MP should now trigger a by-election, Ms Swinson said she looked forward to seeing Ms Wollaston re-elected at a general election - which she expected to happen "in months, if not weeks".
However, she was among six MPs to leave Change UK - which has since become the Independent Group for Change - in June following a disappointing performance in the EU elections.
Leader of the Independent Group for Change Anna Soubry said she would not be following Ms Wollaston in joining the Lib Dems.
She said the Lib Dem leader Ms Swinson had "many of the qualities needed in a prime minister" but added that her own party "offers something that adds to the centrist movement".