The Lib Dem peer Lord Oakeshott has resigned from the party "with a heavy heart" and a damning final attack against leader Nick Clegg.
Lord Oakeshott was widely named as the source of a recent piece of ICM research that suggested the Deputy Prime Minister would lose his parliamentary seat in the general election next year, and stood down after he was told he would face disciplinary action as a result.
Going out in a blaze of criticism for Mr Clegg and his "dire approval ratings", the peer said the Lib Dems were "heading for disaster" without an urgent change to the leadership.
Earlier, Mr Clegg had described Lord Oakeshott's actions as "totally unacceptable" and said that "appropriate steps" would be taken when Parliament returns.
The publication of the ICM poll came amid a crisis in the Lib Dem leadership, with a disastrous result in the European elections prompting calls from the grass roots of the party for Mr Clegg to stand down.
It further fuelled speculation that the Business Secretary, Vince Cable, was preparing to launch his own leadership bid.
Last night Mr Cable provided his own statement on the matter, condemning Lord Oakeshott's behaviour as "inexcusable and reprehensible", and repeating his view that there is "no leadership issue".
Speaking after he delivered a speech in east London, Mr Clegg said: "I think it is totally unacceptable in a campaign for the Liberal party facing very, very difficult elections last week, as we were, to find out now with hindsight that a senior member of the party, far from actually going out trying to win votes was spending money and time seeking to undermine the fortunes of the party."
He said the ICM results had been "confounded" by last week's election results, which suggested that - despite recent difficulties - he would have won his constituency.
Issuing a statement today, Lord Oakeshott said: "I am today taking leave of absence from the House of Lords and resigning as a member of the Liberal Democrats.
"I am sure the party is heading for disaster if it keeps Nick Clegg; and I must not get in the way of the many brave Liberal Democrats fighting for change."