Conservative pensions minister Baroness Altmann has been expelled from the Labour Party after it was found she had been a member for 18 months.

David Cameron made the ex-Saga director a peer so she could become a minister.

But The Huffington Post website revealed Baroness Altmann had been a Labour member for four months after taking up the role and was offered a vote in the party's leadership contest.

She told the BBC she had also renewed her Lib Dem membership in March.

The peer is understood to have renewed her Labour membership at the same time, despite working as a "business champion" for older people for the Conservative/Liberal Democrat coalition.

'Basic checks'

A spokeswoman for the minister said she "has taken an interest in all three parties" because of her previous role as director of over-50s group Saga and had previously acted as an adviser to former Labour Prime Minister Tony Blair.

"She is now only a member of the Conservatives," added the spokeswoman.

Analysis, by BBC political correspondent Chris Mason

It's emerged tonight that the coalition government may have carried on after the general election - after Tory minister Ros Altman told the BBC she renewed her membership of the Lib Dems as recently as March.

The pensions expert - who was offered a peerage by David Cameron in April - also renewed her membership of the Labour Party at the same time.

Her spokeswoman told the BBC she joined all three parties in March 2014 to help her work as a pensions campaigner, and that these memberships were maintained and up to date.

Once it was pointed out to Her Majesty's Opposition that a sitting government minister was a member of their party she was immediately expelled.

And whilst it is reasonable to assume Lady Altmann is no longer a rank and file member of Tim Farron's Lib Dem party, it is possible she still was, whilst serving as a Conservative minister.

So the legacy of that Rose Garden moment with Messrs Cameron and Clegg may just have lasted longer than five years.

The Labour Party, which has been under fire recently over the verification process for people wanting to take part in its leadership election, expelled the Tory minister as soon as the anomaly was discovered.

A Labour source claimed the Conservatives did not carry out "even basic checks" on their ministerial appointments.

But a Conservative representative suggested it was Labour that should be embarrassed for offering a well-known Tory minister a vote in its leadership election.

A spokesman for the Lib Dems said that due to "data protection rules" he could not confirm "who is or isn't, was or wasn't" a party member.

But the spokesman pointed out that anyone campaigning or standing against their party, "let alone being in government", would no longer be able to continue as a Liberal Democrat member.