The initial three-mile section of the underground rail link should have been opened last Friday but London Underground admitted that it could still be weeks away.
London Transport chief executive Denis Tunnicliffe said in November the first section of the line from Stratford, east London, to North Greenwich, the station underneath the Dome, would open by the end of April.
But London Underground has not even asked railway inspectors from the Health and Safety Executive (HSE) to start the process of approving the line's safety regime - known as the "safety case" - raising fears that there are safety worries over the line.
The delay threatens to have a knock-on effect on the other sections of the line. Phase two, between North Greenwich and Waterloo, is scheduled to open by 31 August, and the final link to Green Park in central London by 31 October, just eight weeks before the start of the new millennium.
An LU spokeswoman said the opening of the first phase was "imminent". "We hope to have it open within the next couple of weeks but the inspectors will want to be all over it. We don't know what they will say."
Her Majesty's Railway Inspectorate, part of the HSE, is in constant contact with LU but cannot start the formal approval process until asked.
Mr Tunnicliffe told MPs on the Culture, Media and Sport Committee last year that LU planned to open the first phase "by the end of April 1999", although Tube bosses have since adjusted that to "late spring".
The chairman of the committee, Gerald Kaufman, said he would raise the issue at a private meeting of members of the committee, which now plans to launch a fresh inquiry into transport links to the Dome. "This is obviously a matter of concern," he said.
A spokeswoman for the Department of the Environment, Transport and the Regions, said: "We understand that the deadline is `late spring' and that's what they are working towards.
"I would expect that before any trains run that the proper safety procedures would have gone through."
The latest slippage will increase pressure on London Underground and the project manager, American engineering giant Bechtel, to speed up the work.
They have already agreed to pay bonuses of up to pounds 2,000 to the 600 electricians for completing the job on time.
This has prompted construction workers to demand rewards of up to pounds 3,000 - raising the prospect that LU and Bechtel will be held to ransom by its workforce.
The delay is the latest in a series of setbacks to have hit the beleaguered project, which was originally scheduled to open in spring 1998 and cost pounds 2.1bn.
The project has been bedevilled with financial, industrial and technical problems. In 1992 Olympia & York, the property developer behind Canary Wharf, which would have paid pounds 400m towards the work, went into receivership.
In October 1994 work was halted after a section of tunnel at Heathrow Airport, built using a similar method, collapsed. Work was held up for about a fortnight last year after the electricians launched "wildcat" strikes.
The cost of the project has also soared. Back in 1989, LU estimated the cost of the JLE at around pounds 1bn.
By the time work began in 1993, the budget was pounds 2.1bn. Last autumn the cost had risen to pounds 2.85m and is now running at pounds 3.3bn.