Former First Minister Alex Salmond urged Labour to “find a backbone” and to “become an effective opposition” after they abstained on a vote that could have seen the Tory Government suffer its first defeat of the new parliament.

Parliament was voting was on an amendment to the EU referendum bill that would have introduced a period of purdah.

Purdah is normal practice before elections and referendums. It means that governments are not able to introduce any new policies or initiatives in the four weeks leading up to a vote. It was imposed before the referendum on Scottish independence last year, although Westminster parties were accused of breaking it while campaigning as part of the Better Together campaign.

David Cameron had wanted to avoid purdah before the European referendum, claiming it would interfere with the working of government.  This move sparked a backbench revolt among Tory MPs. The SNP argued that it would have no effect on an “impartial” government.

Cameron had earlier avoided a defeat over suggestions that the European referendum be held on the May 5, the same day as Holyrood and local government elections. When it became obvious that his party would not back his preferred timing of the referendum, Cameron changed plans before they went to the vote.

Despite attempts to persuade his backbench rebels to hold off, the purdah proposal was taken to the vote.  Cameron won by 288 votes to 97. Had Labour voted with the SNP it would have seen him defeated.  Instead they abstained.

Salmond, who is the SNP’s foreign affairs spokesperson, said: “It is almost unbelievable. It was a penalty kick with an open goal ... and Labour did not even kick the ball.

‘‘Labour have yet again chosen to abstain on a key vote – they need to find a backbone and become an effective opposition in parliament.

“The UK Government has already caved on its proposal to have the EU referendum on the same day as the Scottish and other elections – but now because of Labour, we’ve missed the opportunity to defeat the government on purdah restrictions. It was clear that the Tories were angling to hijack Scottish elections with the EU referendum and the united opposition had blown them off course.

“The SNP will continue to campaign for Scotland’s role in the EU – but if we are to have an EU referendum we will help ensure it’s done to the gold standard of the Scottish referendum.”

Speaking in the Commons, Europe minister David Liddington said: “It is vital that the British public and both sides in the referendum debate accept the referendum is being conducted fairly and that therefore everybody will feel they are able to accept the result”.

Liddington added: “It is not our intention that the Government should be a lead campaigner in the referendum and it is right the House should seek reassurance from us on this point.”

Salmond pointed to the referendum on Scottish independence: “Let’s take a scenario, the possibility, that at some point in the course of the referendum campaign next year or the year after that, the No side moves to the front of that referendum.

“In order to try and get the result Prime Minister Cameron would wish of that time, that he wants a Yes result, he needs a last-minute initiative.

‘‘And with no rules or restrictions saying new political initiatives should not be made at a government level in the last 28 days of the campaign, what would be to stop such a prime minister at that time doing a tour of European capitals... suspending question time in the national parliaments and flying as one here to London to announce a new commitment, a new undertaking, a new pledge – a new vow?”

At least 27 Conservative MPs are thought to have voted against the Government. If they were joined by all opposition MPs, there would have been enough to defeat David Cameron’s Government, which currently has a working majority of 16.