Mr McMillan-Scott fell out with the Conservative leadership
An ex-senior Tory MEP, expelled in a row over the party's new allies in Europe, has joined the Lib Dems.
Edward McMillan-Scott, who once headed the Tory grouping in Brussels, was unhappy with David Cameron's decision to re-align the party in Europe.
Lib Dem leader Nick Clegg said he was "delighted" to welcome an MEP who was "respected... across Europe".
The Tories said Mr McMillan-Scott was expelled six months ago: "He has not been a Conservative MEP since then."
A senior party source told the BBC: " They're welcome to him."
The row dates back to Conservative leader David Cameron's decision to pull his party out of the main centre-right grouping in the European Parliament, the European People's Party (EPP).
Mr Cameron said the EPP, which includes the governing parties in Germany, Italy and France, had federalist views were incompatible with his party's stance on Europe.
Instead, Mr Cameron formed a new coalition, the European Conservative and Reformist Group, made up of centre-right parties from central and eastern Europe.
Mr McMillan-Scott criticised Michal Kaminski, a member of the new coalition, over reported past links with extreme groups, something the Polish MEP denies.
Mr McMillan-Scott defied party orders to stand against Mr Kaminski for an influential role within the European Parliament - and won. But he was subsequently expelled from the Conservative Party.
As the Lib Dems' spring conference gets underway in Birmingham, the party announced Mr McMillan-Scott would be joining them.
Party leader Nick Clegg said: ""I am delighted to welcome Edward McMillan-Scott to the Liberal Democrats.
"For many years he has fought for human rights and democracy world wide and he is rightly a respected politician across Europe.
"As someone of principle he has refused to cosy up to right wing extremists, despite pressure from the Tory machine."
Mr McMillan-Scott added: "My reasons for joining the Liberal Democrats are that in Nick Clegg they have a leader whom I like, admire and respect. They are internationalists, not nationalists. They are committed to politics based the values of fairness and change.
"From being a liberal Conservative I become a conservative liberal. Most of my family are liberals. I am pleased to join the liberal family."
The Labour Party has also attacked the Conservatives' new allies in Europe - Foreign Secretary David Miliband said Mr Kaminski had been "denounced by the Chief Rabbi of Poland for an anti-semitic, neo-Nazi past".
David Cameron said Mr Miliband should apologise as the chief rabbi had since clarified his remarks.
Shadow foreign secretary William Hague criticised Mr Miliband's remarks as "smears" and accused him of "shoddy politics".