Although officially neither the Government or BAA, the company that owns Heathrow, will admit that T5, as the new terminal is known, is linked to the road scheme, civil servants at the public inquiry into the terminal have stated that no other option exists.
Thomas Dockerty, a Highways Agency official, told Roy Vandermeer QC, the inquiry's inspector, earlier this month that "although T5 is not the cause of the M25 widening, or related to it, nevertheless T5 depends upon it".
The public inquiry has been running since May 1995 and costs pounds 1m a year. The last time the Government altered the M25 scheme - in November 1996, by cutting out the link roads to T5, previously considered essential - the public inquiry had to be adjourned for three weeks while planners reassessed their traffic forecasts. The first phase of the new terminal is to be completed by 2003.
Mr Dockerty yesterday said that the Highways Agency was not considering any other alternative schemes. When asked what would happen should the M25 scheme be dropped, he said: "Clearly we have said that we are not keen to connect to the existing motorway network. We believe that under the current system we could not marshal the volume of traffic."
Donald Anderson, BAA's senior planner on T5, said that the company had not considered an alternative road link and said that if the M25 proposals were deferred it would "not be easy to carry on".
The widening project, which would leave some parts of the motorway with 12 lanes, was expected to be approved yesterday. But Mr Prescott overruled his Cabinet colleague Gavin Strang, the transport minister, who had given the scheme the green light.
It remains unclear whether further work will uncover new alternatives to the road scheme. Proponents of T5 hope that at worst the decision will be put off until next year.
Despite reaching decisions on most of the 12 projects under the Government's "accelerated review" programme, transport ministers have found themselves under attack from environmentalists. It emerged yesterday that the decision concerning another project - the A2/M2 junction improvement - might also be deferred until next year.
The Department of Transport stressed that the final decision had not been taken and an announcement on the M25 and 11 other schemes would come "before the end of the month".