David Cameron's last minute concessions to Tory MPs over his EU referendum bill may be too little, too late as the prime minister still faces a backbench rebellion over the issue.

Europe minister David Lidington promised to all Conservative MPs that the historic vote would not be held on 5 May next year, when the devolved governments will have their elections, and pledged that the government would limit the amount of public money it would spend on its pro-EU campaign.

The all-Tory government made the move after Sir William Cash tabled an amendment to the draft EU legislation.

The change, reportedly supported by around 50 Conservative MPs, would introduce a purdah period in the run-up to the referendum, which would stop civil service resources being used to promote the government's position.

But, despite the concessions from Lidington, a senior Tory MP warned the government that he could still rebel. Dr Liam Fox, a former defence secretary, argued that it was important for the referendum to be seen as fair.

The North Somerset MP added: "It is unseemly at best for the executive to exempt itself from the legal, electoral and constitutional arrangements that it finds inconvenient during any electoral process."

Fox, citing the period of purdah during the Scottish independence referendum just last year, blasted the government's comments on the issue as "self-serving".

"They sound like the arguments of civil servants and lawyers that ministers have been too keen to listen to and under the full glare of scrutiny of this house and the media they come to sound increasingly self-serving," he said.

The former minister stressed that the purdah rules, set out in the Political Parties, Elections and Referendums Act 2000, were important to "stop the government of the day from affecting the independence or the fairness of any electoral process using the machinery of government".

Alex Salmond, the SNP international affairs and Europe spokesman, also criticised the government over the purdah issue. His party have also tabled their own amendment to implement a period of non-civil service involvement.

The former first minister argued that the government's position – that it would not be able to function because of a purdah period – is not consistent. Salmond said: "If it can be done in each and every general election, then it can be done in this referendum campaign."

Meanwhile, Labour have tabled another amendment to the EU referendum bill. The reds have called for a half-way house between the government and the rebels' position.

The change would make sure that the government, before a date is set for the referendum, released a report on what materials it would publish in the four weeks before the vote.

A Labour source told IBTimes UK the condition would enable the government to continue with its normal functions but it would stop Cameron from "pulling a fast one and issuing a [pro-In] leaflet to 40 million people" ahead of the referendum.