Tony Blair has condemned the RMT's strike action but refused to back demands for Labour to sever its links with the union.

Speaking as the rail union threatened to extend its industrial action across the country, Blair said the strikes could not be justified.

"I have made it clear that we consider these strikes unjustified and, indeed, unjustifiable," said Blair. "That is why we condemn them."

His remarks came as Iain Duncan Smith called on the prime minister to say how much the union had donated to Labour.

Claiming that the RMT had given over £84,000 to the the party in the last year, Duncan Smith said Labour should call a halt to accepting money from hardline trade unions.

"This is a union which is causing misery for passengers up and down the country," said Duncan Smith. "While passengers trying to get to work are losing money, his party is still making money for this union.

"It's not enough for him to say he condemns the strikes. Will he now take a lead, take action and sever links with this striking union."

Dismissing Duncan Smith's demands, the prime minister argued that his opposite number was wrong in calling for strike action to be made illegal on the railways.

"The worst thing we can possibly do is to adopt the proposal he made yesterday which was to ban the right to strike," said Blair.