East Coast will be government-owned
The East Coast Main Line is to be renationalised on 12 December, the RMT has claimed.
The union said it had received a briefing on arrangements for a new publicly owned company which was already trading as East Coast.
National Express was told in the summer it would have to relinquish the route.
Both the Department for Transport and National Express have said no date has been set, but agree new arrangements should be in place by January.
The East Coast Main Line provides trains between London and Edinburgh.
A Department for Transport spokesperson said: "National Express Group has announced it will not provide the further financial support necessary to ensure the subsidiary, National Express East Coast, remains solvent when the current funding runs out.
"It expects this to happen around the end of the year, although no date has been set.
"This would be the point at which the government will assume control of the franchise."
A National Express East Coast spokesperson said: "Discussions continue on the hand-back of the East Coast rail franchise.
"We are working towards agreeing a formal hand-back date for the franchise, prior to year end."
The RMT said it had learned that East Coast would start to run the route within a matter of weeks.
"It is scheduled to take over the operation of the route from 12 December 2009," the union's general secretary Bob Crow said.
"The RMT is demanding an absolute assurance that this will be a permanent move that recognises the chaotic failures of privatisation on this prestige route on the UK rail network.
"It would be a total waste of taxpayers' money and staff time and energy to have a third gamble on privatisation in 18 months' time."
East Coast Main Line services pass through Hertfordshire, Cambridgeshire, Lincolnshire, Nottinghamshire, Yorkshire, Durham and Northumberland.