Ros Altmann, the Conservative minister who was discovered to be a Labour Party member, was also a member of the Lib Dems.
Labour officials decided to kick Baroness Altmann out of the party after The Huffington Post discovered that she had been given a vote in the party's leadership election.
It is understood her Lib Dem membership has also been revoked. A party source joked: "Ironically, we’ve have got rid of the last remaining Lib Dem in government."
Altmann said it was all a "big storm in teacup" and she had joined the three main parties to gather information about pensions reform.
On Wednesday, Lib Dems expressed surprise that the Tory peer had been a member of their party as well as Labour and the Tories.
Altmann has long been one of the UK's most prominent pensions reform campaigners and has been courted by all parties over the years.
She has long campaigned for better rates for savers, for better compensation to those who lose out from pension protection and for reforms to women's pensions.
She served as a non-executive policy adviser in Downing Street when Tony Blair was Prime Minister, but was appointed by pensions minister Steve Webb as the Businsess Champion for Older Workers under the Tory-Lib Dem Coalition in July 2014.