Liberal Democrat peer Lord Rennard has given his party until Thursday to lift his suspension or face legal action at the High Court in a legal letter issued at the party's headquarters.
Former Lib Dem chief executive was suspended on January 20 after failing to apologise to a number of women who had made sexual harassment complaints against him despite Nick Clegg telling him: "No apology, no whip".
Rennard's letter is understood to name individuals involved in the suspension decision, including the chairman of the Lib Dem regional parties committee Mike Wheatley and the chairman of the English Council of Lib Dems Peter Ellis.
A party spokesman said: "We have received a legal letter last Thursday. It does not constitute serving legal papers."