Danny Alexander has indicated he signed a pledge not to raise English university tuition fees despite warning Nick Clegg the policy might be unaffordable.

The Treasury Chief Secretary said the LibDems discussed the issue before the 2010 election campaign.

But he signed up to a pledge that all LibDem MPs were asked to make during the campaign itself. The pledge was organised by the National Union of Students, although the party had already signalled opposition to fees in its manifesto.

LibDem leader Mr Clegg has apologised for making a promise his party could not deliver.

Mr Alexander said: "I signed [the pledge] during the election campaign.

"The discussions that you're referring to took place way before the election campaign, but having as a party agreed to include this policy in our manifesto, I followed through on that in the way that every other Liberal Democrat MP did and with all the consequences we know about."

Michael Moore, the Scottish Secretary, said he signed the pledge before he realised it was unaffordable. He insisted he had "not acted in bad faith".

He added: "It was after the election that people had to weigh up a whole range of budgetary choices."

In 2010 the Coalition agreed to raise English tuition fees to £9000.

In response, many Scottish universities raised their charges for students from outside Scotland to almost that level.