Voters now face a "fairer" EU referendum, a leading rebel has claimed, after David Cameron was defeated in the Commons over moves to relax campaigning rules.
A 37-strong backbench Tory rebellion was easily enough to collapse the Government's fragile majority after Labour joined the fight and last-ditch efforts to appease critics failed.
In the first such reverse since the election of a Conservative-only administration in May, the Government was defeated by 27 votes after Labour joined attacks on plans to water down restrictions on state-funded campaigning.
On June, 27 Conservative MPs voted against the Government on a similar amendment - although on that occasion ministers avoided defeat as Labour abstained.
A concession that any changes to "purdah" rules would be in place at least four months before polling day proved insufficient to head off the revolt, with the Government's proposed watered-down version defeated by 312 votes to 285, majority 27.
A Labour amendment reimposing the existing restrictions - which ban the use of public money to promote one side in the final 28 days of a referendum campaign - was passed without a vote.
Shadow foreign secretary Hilary Benn said it was a "humiliating" defeat for a Government that tried to play "fast and loose" with arrangements for the vote on whether the UK should remain a member of the EU, which is due by the end of 2017.
But Bernard Jenkin, who spearheaded opposition on the Conservative benches, said he believed ministers had been the victims of bad advice that the "purdah" rules would impact on day-to-day EU business.
He criticised ministers for apparently accepting the principle that "purdah" should apply but "still trying to dilute it", but blamed officials for the impasse.
"I do not think the Government's integrity is in doubt," he told the Press Association after the vote.
"The Government has concerns but those concerns are groundless. It is absolutely plain that there is nothing wrong with the existing Political Parties, Elections and Referendums Act as it exists."
Press Association analysis of the division lists showed Mr Jenkin was one of 37 Conservative MPs who voted against the Government, including the chairman of the influential backbench 1922 Committee Graham Brady, former cabinet ministers Liam Fox, Cheryl Gillan, David Jones, Owen Paterson and John Redwood, as well as two MPs elected for the first time in May: Craig Mackinlay and Tom Pursglove.
Ukip MP Douglas Carswell said: "This was a good night for Parliament, for Britain and democracy. Team 'leave' is winning the arguments."