Details of the kind of service passengers can expect when a publicly run key rail line returns to the private sector are being laid out today by the Government.

Labour and rail unions have been among those bitterly opposed to the refranchising of the East Coast line which has been run in the public sector for four years.

The RMT union has said that the prospectus being revealed today will see a return to "third-class travel" on the line which runs from London to Scotland.

The RMT and others have also pointed out that the East Coast has proved a real value-for-money line in that it has returned large amounts of money to the Treasury since National Express pulled out of the franchise in November 2009.

Labour has also expressed disappointment that, with the franchise programme on the railways put back and altered following the botched West Coast franchise bidding process, the Government has appeared to prioritise the East Coast refranchising - due in early 2015 - for political purposes.

Shadow transport secretary Mary Creagh said today: "David Cameron should tackle his Government's cost-of-living crisis and cap rail fare rises for struggling commuters, instead of obsessing about handing East Coast over to the private sector.

"East Coast is working well and will have returned £800 million to the taxpayer by the end of this financial year.

"This Government has learned nothing from the West Coast franchising fiasco, which saw more than £50 million of taxpayers' money wasted in compensation to train companies because of ministers' incompetence."

The RMT has said that it understands the East Coast contract would be awarded "on lucrative, profiteering terms that will enable them to reduce the quality of service [and] introduce a new band of third-class travel".

However it is thought that the so-called "third class" will involve an offer of an airline-style "premium economy" service, between the present standard class and first class.

Channel Tunnel high-speed train company Eurostar has said it wants to bid for the East Coast franchise in partnership with French company Keolis which already runs a number of UK domestic lines.

A Department for Transport spokesman said: "This is ill informed scaremongering from the RMT. By choosing to release inaccurate information, they are misleading the public.

"We have no intention of requiring a 'third class' service, reducing performance levels, or in any way devaluing this vital railway. As the independent Brown Review (into franchising) concluded, franchising works for passengers and taxpayers alike."

RMT general secretary Bob Crow said: "It is simply outrageous that the Government are firing the starting gun on the reprivatisation of the East Coast when every objective analysis shows that this is a successful and reliable service contributing a billion pounds back to the Government while the private operators are milking huge profits and soaking up vast taxpayer subsidies.

"The fact that the public option is putting over a billion pounds back into the Treasury, while improving services and delivering more capacity, has been cynically airbrushed over by the Government and their officials is a disgraceful piece of political chicanery which must be called in for scrutiny to hold back this full-tilt dash for privatisation."

He added: "If the Government are allowed to get away with this, passengers can expect third-class services, shoddy performance and higher fares while the private companies get a gold-plated, 11-year franchise that will enable them to launder hundreds of millions of pounds into the pockets of their shareholders.

"It stinks and it must be stopped and the campaign to prevent the reprivatisation will haunt this rotten government right up to polling day."