Tube Lines said they believed the work could finish in December
Contractors upgrading the Tube's Jubilee Line have asked for six further weekend closures to finish the work.
Disruptions to the line have sparked criticism from the London Mayor Boris Johnson and the business community.
Maintenance firm Tube Lines and contractors Bechtel were supposed to have finished the upgrade by December.
Tube Lines said 95% of the work had been done but that reliability testing was needed. London Underground has the final say on closure schedules.
The line has been affected by closure in relation to this project since February 2007.
In August, Tube bosses suggested that the reputation of the two firms hinged on their commitment to delivery.
A statement from Tube Lines read: "The upgrade of the Jubilee line is currently the most technically complex project being carried out on the Tube and will be the first Metro rail line in the UK to be fitted with this type of signalling system.
"It is regrettable that this project cannot be delivered by the end of this year but access to the line is crucial to completing the work."
It went on: "We have submitted to London Underground a number of closure programmes since the summer and estimate that a further six full-weekend closures would be required to complete the signalling testing.
"Once completed, London Underground will also need to be able to access the track to be able to train their drivers on the new system."
Meanwhile Transport for London issued a statement which said: "We don't take this disruption lightly, which is why we've pressed Tube Lines to conduct an independent review.
"It is vital this is completed as soon as possible, so Tube Lines can present us with a clear and credible path towards a revised delivery date.
"It is clear that this will now extend well into 2010 and we will try to minimise the impact, of the further weekend closures that Tube Lines will need, on London."