Portsmouth South MP Mike Hancock has resigned his membership of the Liberal Democrats the party has confirmed.
The move came after the veteran MP, who had already been suspended by the party, was forced publicly to apologise to a vulnerable woman constituent for making improper sexual advances.
In June Mr Hancock gave an "unreserved" apology at London's High Court over an "inappropriate and unprofessional friendship" with a single mother known as Annie, who had sought his assistance as her MP and local councillor.
Solicitors for the woman said the apology was part of a "confidential settlement" in which the "vulnerable woman", who suffers from mental health problems, brought a claim for damages against him.
A Lib Dem spokesman said Mr Hancock had quit before an internal party disciplinary hearing.
Lib Dem leader and Deputy Prime Minister Nick Clegg had previously called on Mr Hancock to resign his membership and said he had "no future" in the party.
A Lib Dem spokesman said: "I can confirm he has resigned from the party. "We have received a letter confirming his resignation. Obviously, there was due to be a disciplinary hearing imminently, so before that has happened he has resigned."