The government is preparing to offer a number of concessions to rebel backbenchers opposing its bill on university tuition fees.

The bill is now unlikely to go before parliament before Christmas in order to giver Charles Clarke more time to win over Labour MPs.

However, one senior aide to the education secretary insisted: "Make no mistake, the fundamental principles in the package are not negotiable because that would torpedo our objectives."

A primary concession on offer to the rebels is offering to raise the level at which graduates start paying back fees and loans from £15,000 to £18,000 or £20,000.

Over the weekend a series of senior Labour figures, including former chief whip Nick Brown, spoke out against the government's plans.