Mr Thurnham resigned the party whip earlier this year. Since then, he said, "things have not got better; they've got worse".
Referring to the row over ex-minister Neil Hamilton, Mr Thurnham added: "I cannot, above all, accept the latest attempts by the Government to suppress enquiries into allegations of sleaze which poison the very basis of our democracy.
"I despair at the weak leadership and Government disunity which weakens our international position at a time when this country needs national purpose and strength."
Mr Thurnham, MP for Bolton North East since June 1983, also condemned calls for "highly irresponsible" tax cuts, when the national debt "has doubled under John Major, and is still climbing".
Mr Thurnham resigned the Tory whip earlier this year, citing qualms about the Government's approach to the Scott report on arms to Iraq. But that decision also followed his failure to secure the safe seat of Westmorland and Lonsdale, which went to a close ally of Mr Major's.
The MP's defection will not have surprised the Tory whips who had failed to lure Mr Thurnham back into the fold.
The Tories reacted coolly to news of the defection. A spokeswoman said: "This doesn't change anything. Our majority is still the same, which is two." It had been one until Labour MP Terry Patchett died last week.Reuse content