The government could face a backbench rebellion in the House of Commons today amid a row over civil service neutrality in the run-up to the referendum on Britain's European Union membership.
The bill paving the way for the referendum will reach its final stage in the Commons on Monday.
But a number of Conservative backbenchers have accused ministers of failing to do enough to ensure that the machinery of government is not used to campaign to keep Britain in the EU, and as many as 30 could side with Labour and the SNP to inflict a defeat on the government.
Ministers had originally sought to make changes to section 125 of the Political Parties, Elections and Referendum Act 2000, which places restrictions – including a 28-day "purdah" period – on the use of government resources in the run-up to a referendum.
The government had argued that the changes were needed to allow normal business to carry on ahead of the vote, but last week foreign secretary Philip Hammond announced a series of amendments designed to reassure backbenchers that purdah could only be suspended with parliament's backing.
Ahead of the Commons vote, Europe minister David Lidington urged MPs to back the government, and sought to explain the thinking behind the latest amendment.
"Philip Hammond and I have been clear that the government should not and will not use public resources to pay for ‘campaigning’ activities in those final few weeks," he wrote on the ConservativeHome website.
"That four week campaign should be a matter for campaign groups, with no government advertising campaigns, glossy mailshots, door-drops, target letters or mass emails.
"But ministers will want to be able to explain the outcome of the renegotiation, set out our recommendation and explain our reasons. Indeed, I think that the British public will expect the prime minister, who initiated and led the renegotiation, to set out his stall to them.
"The government new clause gives parliament a veto on any proposals for flexibility to permit the government to communicate its case. Without a positive vote in parliament, the full rigour of S 125 would stay in place."
But Steve Baker, co-chair of the Eurosceptic Conservatives for Britain group of Tory MPs, has argued that the amendments still give the government "considerable discretion over purdah", while Conservative former environment secretary Owen Paterson accused Downing Street of orchestrating "sneaky little tricks" to try and get the measures passed.
The vote comes amid a report in Monday's Financial Times that British businesses have been warned against making "counterproductive" statements about the risks of leaving the EU ahead of the referendum, which is due to take place by 2017.