The party kicked her out after the Huffington Post discovered she had been given a vote in its leadership election.
Altmann, who has been serving as a minister for four months, had been a fully-paid up member of Labour since March 2014, until the party expelled her yesterday, saying she didn't support its "aims and values". No kidding.
A source asked the Huffington Post: "Is she also a member of the Tories? Are they happy that one of their ministers was a Labour party member? Did they do even basic checks before David Cameron appointed her?"
To be fair, it was already widely known that Altmann had been involved with other parties over the years. Tom McPhail, head of pensions research at Hargreaves Lansdown, said:
Ros advised the Blair government on pensions policy. She had also been a close ally of the Liberal Democrat pensions minister Steve Webb.
[She] was brought into the current government because she has very considerable expertise in pensions and because she has been an effective campaigner for justice on pension related issues. Her political sympathies are less important than whether she can perform a good job as the pensions minister.
The revelation of Altmann's Labour membership came just days before the leadership election result is announced, a campaign that has been fraught with controversy over its new members, and whether the party has been infiltrated.
The Conservative party declined to comment.