One in six London Labour MPs has now publicly expressed doubts about Ed Miliband’s plan to impose a mansion tax on homes worth more than £2m, as Glenda Jackson joined those attacking the policy.

The MP for the marginal seat of Hampstead and Kilburn said it would “impact disastrously on people who are asset rich but revenue poor, particularly pensioners, who bought their houses many years ago and through no fault of their own have seen the value rise because of the ludicrous London house prices”.

Jackson joins fellow London MPs Diane Abbott, David Lammy, Margaret Hodge, Tessa Jowell and Nick Raynsford in expressing reservations about the tax, which Miliband confirmed at Labour’s party conference.

With a London mayoral election coming up in 2016, some within Labour fear it could harm the party’s chances of taking back control of the city when Boris Johnson steps down, lose them votes at the general election. Abbott, Hodge, Lammy and Jowell are all tipped to throw their hat into the ring to be mayoral candidates.

A majority of the public back the idea of a mansion tax, according to opinion polls. Miliband has said the proceeds would be used to help to fund 20,000 new nurses, 8,000 more GPs, 5,000 new home-care workers and 3,000 new midwives.

Labour has also made it clear that asset-rich, cash-poor pensioners would not have to suffer, saying the party would look at a relief scheme or a way to allow those on modest incomes to defer payment until the property was sold.