Mr Winsor believes ministers are routinely putting unfair pressure on his successor, Chris Bolt, harming his ability to act as an impartial referee whose role is to ensure private rail companies are fairly treated. Last year's rail shake-up, which limited the regulator's power to set the running costs of the network, was a step too far. Further "unjustified encroachments" will "set off very serious alarms about the incremental renationalisation of the railway and the continued diminution of the role of the regulatory authority".

His comments, in an article for Rail Professional magazine, are his most outspoken criticism of ministers since his five-year term as rail regulator ended in 1994. He believes there is a danger of the regulator's independence being lost for good, concluding: "That really would be the end of the privatised railway."

Now a partner at the law firm White & Case, Mr Winsor has advised new entrant Grand Central Railways on its controversial bid to operate London-to-Sunderland services which has been approved by the regulator despite fierce opposition from ministers. Grand Central trains will stop at York and will offer a flat return fare of £55 between Yorkshire and the capital, which the incumbent inter-city operator GNER says will eat into its market.

The transport secretary, Alistair Darling, fiercely opposed Grand Central's bid because any damage to GNER could hamper its ability to pay £1.3bn in profit premiums to the government over a decade. In an exchange of letters released under the Freedom of Information Act, government officials told the rail regulator that any approval of Grand Central would unsettle train operators across the industry, leading to more caution, lower bids for franchises and higher government subsidies.

Government officials accused the regulator of applying an incomprehensible rationale in its approval of the bid, giving a lack of detail and withholding co-operation. GNER is angry about Grand Central's imminent arrival. GNER's chief executive, Christopher Garnett, told Rail Magazine this week that he considered the rail regulator to be "stupid", "arrogant" and "incompetent".

A government spokesman yesterday described Mr Winsor's comments as "complete rubbish".