The idea of third-tier services on the Edinburgh-to-London King's Cross service harks back to the era of steam trains 50 years ago.
The return of the idea, last seen in 1956 on Britain's railways, has been mooted in a draft prospectus for would-be private bidders interested in taking on the franchise from the Government.
But ministers were accused of turning the clock back half a century after the proposals for returning the franchise to private hands in two years time were obtained by the Rail, Maritime and Transport union (RMT).
The document gives bidders the option of establishing an "intermediate class between Standard and First", paving the way to three classes of travel on East Coast.
The Government has said the move was intended to create a carriage on a par with premium economy on airlines. However, critics it was more likely to be on a par with the no-frills industry.
Bob Crow, leader of the RMT Transport Union, which opposes reprivatising the line, said: "Twenty years of experience of private operators points to a Ryanair model with passengers rammed in to creaking cattle trucks with a surcharge for everything from your bags to using the lavatories.
"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 is unclear how third class might work, but suggestions have included a reservation-only area guaranteeing every passenger a seat, or greater legroom.
East Coast has been in the public sector since autumn 2009 but will return to private ownership in early 2015.
The union also claims the prospectus redacts sections highlighting the good punctuality and performance of the service under public ownership, including a section about the East Coast being the carrier of a higher number of passengers per train mile than any other franchised UK operator.
They also warn of possible service cuts, with the prospectus saying the Government will set only minimum levels, with bidders being able to vary specified first and last trains. Mary Creagh MP, Labour's Shadow Transport Secretary, said: "David Cameron says we're all in this together but if that's true then why is he going back to the 1950s and reintroducing third class? East Coast passengers deserve better than this."
The Department of Transport accused the RMT of "ill-informed scaremongering".
He added: "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."
l ScotRail and Network Rail's £11 million electrification of the Glasgow Central to Paisley Canal line has won Partnership of the Year at the National Transport Awards.