Steven Byers was under siege last night after he was accused for the second time in 24 hours of having misled Parliament.

The Transport Secretary told the Commons on Monday that he had not threatened to strip the rail regulator, Tom Winsor, of his powers if he attempted to bail out the beleaguered infrastructure company, Railtrack, with more taxpayers' money.

Yesterday however, Mr Winsor insisted to MPs that Mr Byers had indeed warned him that he would introduce emergency legislation to remove the powers if Mr Winsor offered Railtrack any further financial help.

The latest revelations followed Tuesday's observation by John Robinson, the chief executive of Railtrack, that Mr Byers' account to Parliament of how the infrastructure company had accepted it should go into administration was at odds with his own notes on these events.

Mr Winsor's assertions were seen as more damaging than even Mr Robinson's remarks, and senior Tories predicted that the Transport Secretary would be forced to resign.

Mr Byers came under severe pressure recently for refusing to dismiss Jo Moore, his spin doctor, after she sent out the notorious memo on 11 September trawling for bad news the department could "bury''.

Under pressure from Tory MPs on the Commons Transport sub-committee yesterday, Mr Winsor refused to say directly that Mr Byers had "misled'' Parliament. "He might not have regarded his statement as a threat,'' the regulator said.

Senior Conservatives, however, were not so reluctant. Theresa May, the shadow Transport Secretary, described Mr Winsor's arguments as "a startling new piece of evidence'' on top of allegations that Mr Byers had already misled Parliament. "He is a serial offender. His position is now untenable,'' Ms May said.

The Department of Transport, Local Government and the Regions said My Byers did say to Mr Winsor "in passing conversation that he had the power to bring in emergency legislation ... But at no point did he threaten to carry that out because at no point did the company go to Mr Winsor and ask for an interim review."