PFI bidding was due to close at the end of 2008, but the Connect Plus consortium of Balfour Beatty, Skanska, Atkins and Egis Projects revised the deadline late last year to early 2009.
NCE now understands that close will not be reached until the next financial year at the earliest, with five of the 11 lending banks still to commit.
The Highways Agency confirmed that part-funding by the Treasury was one option under consideration to plug the cash shortfall, but as it stands the gap is still is too large.
Speaking at NCE's Road Summit in London today, Highways Agency chief executive Graham Dalton claimed that delays were not yet an inevitability: "There is a plan B [if work does not start in April] - but we're not intending to use it."
He added that the April start date had been determined by the need to complete the widening work in time for the start of the London 2012 Olympic Games.
However, he suggested that there was some "wriggle room" with the completion date and that it might be acceptable leave some of the widening until after the Games, while puting a stop on the works during the event.
The delay in the close of bidding has forced the Agency to extend M25 managing agent Sphere's contract by four and a half months to September.
"This will make sure operation and maintenance of the motorway is not impacted on by a sudden handover (to the PFI contractor)," said an Agency spokesman. "The extension will allow sufficient time for the existing managing agent to plan and carry out maintenance work and ensure a proper handover."
Another Agency spokesperson said: "The M25 DBFO contract is not on hold. We remain confident of a successful outcome, and start of work on widening the M25 is still programmed for April 2009." Connect Plus began advance works in December.
One option being suggested by Campaign for Better Transport is to scale back the project, which splits into four separate widening packages and a long-term maintenance deal.
Sections 2 and 5 are already expected to be tackled with the cheaper hard shoulder running approach (NCE 24 April 2008), and it could be an option to consider the same approach for the two remaining sections.
Campaign for Better Transport executive director Stephen Joseph told NCE: "We think this is a great opportunity to re-look at the scheme and consider [more] hard shoulder running.
"The scheme as it stands is very, very expensive and that money could go a long way to a more cost effective transport scheme such as high speed rail."
Hard shoulder substitute?
The DBFO will widen within the existing land-take of four sections of the M25 from three lanes to four. The sections are:
Section 1: Junctions 16 to 23
Section 2: Junctions 5 to 7
Section 4: Junctions 27 to 30
Section 5: Junctions 23 to 27