Buses and roads took the strain of last week's Tube strike

Shop stewards from the Rail, Maritime and Transport (RMT) union said if a new offer did not arrive by 16 July, dates for new strikes should be set.

Talks between the union and managers broke down on Thursday afternoon.

Bobby Law, the union's London regional organiser, said: "We are disappointed at the lack of progress at the talks."

Mr Law added: "It is clear that management have had their hands tied by the mayor and there is no room for constructive negotiations."

A LU spokesman said the company had re-submitted its deal which is worth 6.75% over two years and the guarantee of a 35-hour week by 2006.

"The RMT reiterated its desire for a four-day week which we said was not affordable. Further meetings are planned."

The journeys of millions of Londoners were disrupted last Tuesday and Wednesday when RMT members took part in a 24-hour walkout.