Three Liberal Democrat frontbenchers quit tonight in a revolt over the Lisbon Treaty referendum.
Justice spokesman David Heath, countryside spokesman Tim Farron and Scotland and Northern Ireland spokesman Alistair Carmichael resigned after defying the leadership to vote for a Commons amendment calling for a referendum.
Party leader Nick Clegg said: "I greatly regret the loss of David, Tim and Alistair from the shadow cabinet. They have served with great distinction and commitment.
"However, as they have recognised, the shadow cabinet cannot operate effectively unless the principle of collective responsibility is maintained."
It is thought that they were among 13 Lib Dem MPs to rebel against the leadership's call to abstain on the amendment tabled by the Conservatives.
Mr Clegg moved swiftly to reassert his battered authority by reassigning the responsibilities of the three frontbenchers.
Home affairs spokesman Chris Huhne will take on justice, environment spokesman Steve Webb's environment team will take on the countryside brief, and international development spokesman Michael Moore will take on Scotland and Northern Ireland.