The Liberal Democrat peer at the centre of a row over sexual harassment allegations has sent a legal letter to the party, giving it until Thursday to lift his suspension or face action at the High Court.
Former Lib Dem chief executive Lord Rennard was suspended on January 20 pending an investigation into whether he had brought the party into disrepute by failing to apologise to a number of women who had made complaints against him.
A QC's inquiry into the allegations concluded that the women's claims were credible but there was insufficient evidence to bring disciplinary charges, and suggested that the peer should make an apology, prompting Lib Dem leader Nick Clegg to tell him: "No apology, no whip."
Lord Rennard offered to take part in mediation with the party and the complainants, but also instructed a QC specialising in public law to advise him on the "lawfulness or otherwise" of the decision to launch a second inquiry into his conduct.
The letter before action served on party HQ 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."