The Transport Secretary is being accused of "personally propping up" failing rail franchises amid reports that a leading operator could be about to lose its contract.
Govia Thameslink Railway was said to be "drinking in the last chance saloon", according to an unnamed Government source quoted by the BBC.
The company's services have been disrupted since new timetables were introduced in May.
Its chief executive Charles Horton has resigned and there have been calls for Transport Secretary Chris Grayling to quit.
Mick Cash, general secretary of the Rail, Maritime and Transport union, said: "Telling these basket-case companies that they are drinking in the last chance saloon is empty rhetoric from a Government ideologically wedded to privatisation and just lets them off the hook again while services are reduced to chaos.
"Chris Grayling has personally propped up Britain's failing rail franchises while RMT members have been deployed at the sharp end as human shields to bear the brunt of the public anger, threats and abuse.
"It's a shocking and cowardly way to treat the rail workforce.
"This is no time for empty threats from unnamed Government sources.
"These rip-off rail companies should have the bottle snatched from them, should be kicked out of the saloon and should be barred for good.
"The only alternative to the crisis on Britain's privatised railways is public ownership on a permanent basis."
Mick Whelan, general secretary of the train drivers' union Aslef, said: "GTR has, not for the first time, badly let down passengers and staff with its inept introduction of a new timetable.
"But the Department for Transport (DfT) too was at fault for creating the GTR super franchise in the first place.
"They have been working hand in glove and are both to blame.
"One of the problems, still to be addressed, is not so much where in these new timetables trains will stop, but where they won't."
A DfT spokesman said: "GTR passengers are encountering unacceptable service levels and the Transport Secretary and Rail Minister have been clear it is their priority that this is put right as soon as possible and passengers are compensated for the disruption.
"An independent inquiry will now be carried out to determine how this went so badly wrong and to make sure this does not happen again.
"We will investigate whether GTR have breached their contracts and we won't hesitate to take tough action against them if they are found negligent."