Tory rebels have joined with Labour to defeat the government over rules designed to ensure civil service impartiality in the upcoming referendum on Britain's membership of the European Union.
Ministers had wanted to introduce various exemptions to the normal rules that stop the machinery of government from being used to publish "promotional material" in favour of one side in an election or referendum.
But Conservative backbenchers on Monday night joined with Labour and the SNP to defeat the Government by 312 votes to 285, in spite of a move by ministers to water down their initial proposals.
The so-called "purdah" regulations are in place 28 days ahead of general elections and were also used before the Scottish independence referendum last year.
The change to the bill was supported not only by Conservative MPs, but also the SNP's former leader Alex Salmond, who told colleagues that even though he was "pro-European to my fingertips" he worried about the vote being tarnished if it was not seen to be a fair campaign.
The government also accepted an amendment tabled by senior Tory backbencher Bernard Jenkin, meaning there must now be at least four months' notice before the date of the vote is announced.
Jenkin, who also chairs parliament's Public Administration and Constitutional Affairs Committee, said the vote was "a victory for a fair referendum".
Ahead of the summer recess, Jenkin's committee took evidence from figures including cabinet secretary Jeremy Heywood, who told MPs that the proposed purdah rule change was needed to allow the normal business of government to continue in the run-up to the vote.
The committee has also called for changes to the Civil Service Code – the rules governing the conduct of officials – "to include specific reference to impartiality and proper conduct during a referendum campaign".
Speaking to the BBC on Tuesday morning, the Conservative backbencher said he still had concerns over the role EU institutions and political parties might play in seeking to sway the vote.
"Let's not have any illusions here; the whole thing is very much still stacked in favour of staying in the European Union," he said.