Rail unions have launched a fresh attack over plans to sell Britain's stake in the Eurostar rail service as the Government prepares to announce the next move in the bidding process.

The Rail, Maritime and Transport union said selling the UK taxpayers' 40% stake was an "act of betrayal".

Unions expect the Government to set out the next phase of the sell-off within days, but there was no official confirmation.

The Eurostar service has stations at Ebbsfleet International and Ashford International in Kent, with a UK terminus at St Pancras.

RMT general secretary Mick Cash said: "This is a gross act of betrayal of the British people by a right wing government hell bent on selling off the family silver regardless of the real cost.

"This compounds the issue of foreign ownership of Britain's railways as the French state have first refusal on our slice of the highly profitable Eurostar cake. The French and Belgians think we are insane knocking off such a valuable and strategic infrastructure asset.

"This sell-off is just a short sighted act of industrial vandalism based on a bankrupt pro-privatisation ideology. RMT will fight this tooth and nail."

The Government put the stake up for sale last December as part of a plan to privatise state assets.

The French state train company SNCF owns 55% of Eurostar and Belgium's SNCB holds 5%.

Mr Cash added: "With Eurostar a strong contender for the resale of the East Coast franchise we could easily end up within the next few years with the French state having complete control of rail services from Paris to Inverness."