David Cameron's EU renegotiation is approaching its moment of truth as he battles to win over fellow leaders and Cabinet colleagues.
The Prime Minister will embark on a final diplomatic push before a crunch Brussels summit where he is hoping to secure a new membership package.
But just as vital is convincing senior government figures to support the deal, amid reports that Attorney General Jeremy Wright is considering backing Leave in the looming referendum campaign.
Justice Secretary Michael Gove is thought to be among a handful of Cabinet ministers wavering, while London mayor Boris Johnson has declared he will make his views known with "deafening eclat" when an agreement is sealed.
Mr Cameron is also coming under pressure to free Eurosceptic ministers from collective responsibility quickly after striking a deal.
Graham Brady, chairman of the influential Tory backbench 1922 Committee, insisted the Cabinet should be gathered on Saturday or even Friday night after the conclusion of the summit.
"I think that is absolutely essential," he told Sky News' Murnaghan programme. "It is in everybody's interests to do this as quickly as possible and if it were to appear that David Cameron was seeking to have the whole weekend to himself to put one side of the argument, I think that would look bad for the Remain campaign."
The two sides are already ramping up their rhetoric, with little more than four months until the expected date of the poll.
Foreign Secretary Philip Hammond said discussions over the membership terms would run "right to the wire" of the leaders' summit this week.
Mr Hammond, who is attending a foreign ministers meeting in the Belgian capital later, argued that the Government had already secured an exemption from "ever-closer union" and a "major breakthrough" on restricting migrant benefits.
And he delivered a stark warning that Britain is likely to be punished by the EU if it chooses to leave, as other states will not want it to "succeed" outside the group.
"People who say we would do a great deal if we left forget that the countries remaining in the EU will be looking over their shoulder at people in their own countries saying, 'Well, if the Brits can do it, why can't we'," he told the BBC's Andrew Marr Show.
"They will not have an interest in demonstrating that we can succeed outside the EU."
Writing in the Sunday Times, easyJet chief Carolyn McCall suggested Brexit could herald a return to the days when flying was "reserved for the elite".
Peter Long, former boss of the Tui travel group that owns Thomson and First Choice, insisted close co-operation with other EU states was essential to "protect the security of our holidaymakers".
The Stronger In campaign has highlighted a poll by German think-tank the Bertelsmann Foundation suggesting 29% of British and German businesses surveyed would either reduce capacity or leave Britain altogether after a vote to leave the EU.
Liam Fox, the Eurosceptic former cabinet minister, hit out at "scaremongering" by the In campaign. "Those that wish to remain in the EU should make the positive case for the supranational European project rather than frightening people," he said.
Vote Leave spokesman Robert Oxley said: "It's such a shame to see pro-EU voices resorting to negative campaigning tactics based on little more than fear and falsehoods."