The warning from London Transport chief executive Denis Tunnicliffe came just weeks after London Underground promised the entire extension would be linked up and working before the end of 1999 - "no question".

The new line will eventually link the dome to central London

Although test trains are running on the new line from Green Park to the dome station at north Greenwich, they are not yet carrying passengers because of continuing problems with two stations along the route.

The prestigious new Westminster station by the Houses of Parliament - currently being upgraded to connect to the new line - is running behind schedule after a series of strikes by electricians working there.

Mr Tunnicliffe said it was not certain the station would be open by 31 December.

Just a 'very little' blow

Giving evidence to MPs on the Commons Culture Select Committee, he said such a failure would represent a "very little" blow to passengers trying to reach the dome, or going to work at the Canary Wharf office complex in east London.

"There are clear and obvious alternative routes," he said, but "in terms of pride it would be a significant blow".

Last month Derek Smith, London Underground managing director, insisted that the entire extension, from Stanmore in north west London, via the dome to Stratford, in east London, would be ready in time for New Year's eve celebrations.

"No question - it will be there, linked up, working," he insisted.

Signal problems

In his evidence on Wednesday Mr Tunnicliffe also told the committee the opening of Southwark station was also running behind schedule - but he hoped to see that completed by the end of October.

He said signalling systems were causing problems along the line and a more "robust" system was needed.

"We believe we will have a robust system running by the millennium," he told reporters afterwards.

Another headache for dome planners is the river boat service to Greenwich.

One of the four piers on the route, at Blackfriars, was behind schedule but LT's Clive Hodson, in charge of river services, said he had "no reason to suspect it will not open" in time.

Mr Tunnicliffe told the MPs the budget for transport to the dome had soared even further over the original estimate.

Projected costs were now running at £3.5bn, up from an earlier revised estimate of £3.2bn.