Plans to loosen rules restricting Government activity in the run up to the EU referendum have been condemned by MPs, who warned that it would "cast a shadow of doubt" over the poll.
The Government has claimed that ministers could be hampered in summits with European counterparts unless the purdah rules are eased in the weeks ahead of the in/out vote on membership of the EU promised by the end of 2017.
But a Commons select committee which looked at the issue concluded that the plan would make it appear as though the Government was "seeking to circumvent proper processes".
The controversial plan has left eurosceptics furious because they fear it will allow Whitehall to support a campaign to stay in the EU, if that is what Prime Minister David Cameron recommends.
Bernard Jenkin, chairman of the Public Administration and Constitutional Affairs Committee, said the Government had to "conduct itself properly, fairly and impartially" during the purdah period.
In a letter to Europe Minister David Lidington, Mr Jenkin said the Government's plan to ditch Section 125 of the Political Parties, Elections and Referendums Act, which covers activity in the 28 days before the poll, was "completely unacceptable".
"While the members of my committee have different views on the EU, we are unanimously of the view that, in respect of any referendum, the Government of the United Kingdom must be seen to conduct itself properly, fairly and impartially during the purdah period," Mr Jenkin said.
"The disapplication or dilution of Section 125 would make it appear that the Government is seeking to circumvent proper processes to enable it to use the machinery of Government for campaigning activity as well as legitimate Government activity in the run up to the EU referendum."
He added that Section 125 was aimed at preventing ministers using the "machinery of government" to explain the outcome of the renegotiation process with Brussels ahead of the referendum "in order to avoid the Government giving a huge advantage to one campaign or another".
The Government has insisted it will not use the change in the rules to campaign or send out leaflets in support of one side or the other.
Mr Jenkin told Mr Lidington: "While we accept your good faith that it is not your intention, the Government's proposal has cast a shadow of doubt over the propriety of the process, even at this early stage.
"We regard this as completely unacceptable. This must be remedied at the report stage of the EU Referendum Bill, so that the integrity of the process and the legitimacy of the result is beyond question."
Senior Tory Mr Jenkin told BBC Radio 4's Today programme the current plans "stacks it incredibly in favour of the Government" because ministers would be able to rely on the support of thousands of press officers and scores of special advisers.
"They are all going to be spin doctoring," he said. "In purdah during a general election, during the Scottish referendum, they had to go offline in the last 28 days and that makes it just a little bit fairer.
"It's likely that the Yes vote will vastly outspend the No campaigns anyway, but this just tilts the playing field even more in favour of the Government's view."