In a letter to Conservative MPs ahead of a crunch Commons vote, Europe minister David Lidington stressed he was ready to “engage” with those who had strong feelings on the issue and bring forward amendments to the legislation at a later date.
But he reiterated that it would be “unworkable and inappropriate” to impose the standard 28-day period in which Whitehall cannot get involved in debate.
“Working out a system that will reassure colleagues and voters that the referendum is a fair fight, yet will preserve the Government’s ability to act in the national interest is not straightforward. It is important that it is legally clear and robust,” Mr Lidington wrote.
“Therefore, we will work with colleagues over the next few months to understand their specific areas of concern and bring forward at report stage in the Autumn government amendments that command the widest possible support within the House and put beyond any doubt that the campaign will be conducted throughout in a manner that all sides will see as fair.”
The message was immediately rejected by Tory rebels, who said they would still push the matter to a vote in the House.
“This is a very simple issue of principle,” Bernard Jenkin told the BBC’s Daily Politics. “If they do not accept the amendment we will push it to a vote.”
Another rebel ringleader, Owen Paterson, said: “We need proper purdah.”
A significant rebellion would be highly embarrassing for David Cameron just six weeks after his general election victory.
However, the chances of the government being defeated has reduced after Labour indicated it is likely to abstain in any purdah vote.