Pensions minister Ros Altmann said she joined all three major UK parties to keep an eye on each’s politics before the election

Ros Altmann, the Conservative pensions minister who was expelled from the Labour party this week, has also been revealed as a recent member of the Liberal Democrats.

Altmann, who was made a peer by David Cameron after the election, claimed she had joined all three main parties as an “independent policy adviser to keep an eye on each party’s politics in my areas of interest ahead of the election”.

It is understood that her Liberal Democrat membership has also been revoked. The party would not confirm that she was a member, citing data protection issues, but a Lib Dem source said: “It’s a bittersweet irony that we’ve had to chuck out the last Lib Dem in government.”

Defending her memberships, Altmann said she had never hidden who she was and Labour had not picked up on it despite the party’s “scrupulous” checks during the leadership election.

Ros Altmann (@rosaltmann) My work on pensions, savings, retmt always focused on policy, not politics. As an independent I wanted to know all 3 parties policies

It emerged on Tuesday that she had been a Labour member for about 18 months and was eligible for a vote in the party’s leadership election. The Tory minister said she had not voted for the next leader and regarded the issue as a “big storm in a teacup”.

A spokeswoman for Altmann said she “has taken an interest in all three parties” because of her previous role as the director of Saga and as a business champion for older people under the last coalition government, and “wanted to keep in touch with what each party was doing”. She added: “She is now only a member of the Conservatives.”

Altmann previously served as an adviser to Tony Blair when he was the prime minister, and is one of the UK’s most prominent pensions reform campaigners. She has pushed for higher rates for savers, better compensation for those who lose out from pension protection and changes to women’s pensions.