Crunch talks to thrash out key Brexit issues such as customs and the Irish border were set to take place this morning.
Cabinet ministers were en route to Chequers early on to try to thrash out their stance on Brexit, amid rifts in the Conservative party.
The meeting will be so serious that special measures will be taken, ITV News Political Editor Robert Peston reported - including the confiscation of mobile phones.
The Prime Minister was said to be keen to ensure that the outside world did not get wind of the negotiations before they were finalised, since the outcome of the Buckinghamshire meeting will be central to the UK's negotiations with the EU.
Previous suggestions over customs arrangements have failed, meaning that Theresa May will suggest a third option on Friday.
This is likely to consist of:
A “facilitated customs arrangement” which would see technology used to determine where goods arriving into the UK will ultimately end up. This would allow the correct tariff to be paid – either at the UK or EU rate.
A "common rule book" for all goods, including agricultural products, with consequences for any divergences, but leaving the UK free to set its own tariffs for countries outside the EU, allowing some free trade deals.
Once the Cabinet has agreed on their proposal for a customs arrangement, they will then publish a white paper - a policy document that set out their proposals for future legislation.
The Prime Minister's "third way" plan is a compromise designed to secure support from both wings of her Cabinet, yet there are already signs many Brexiteers are not happy with it.
Meanwhile, Labour has created a pun-filled away-day survival kit to taunt the Tories while they are locked away at the prime minister's Chequers country retreat.
Inside, Cabinet members can find many tongue-in-cheek options such as plasters, to patch over any disagreements and pyjamas, just in case they end up stuck there all night.
Peston reported that Boris Johnson, Michael Gove, Penny Mordaunt, Andrea Leadsom, Liam Fox, Esther McVey, Chris Grayling, and David Davis met on Wednesday and Thursday to discuss a response to the Prime Minister's plan.
David Jones, a pro-Brexit former minister at the Department for Exiting the EU (DexEU) said the deal looked "not very good at all".
He revealed to the Today programme that it appeared all three "red lines" of leaving the customs union, single market and jurisdiction of the European Court of Justice would be breached by what was being proposed.
Peston reported that International Trade Secretary Dr Fox has already met with Mrs May over the plans, but has been promised that it will not restirct the ability to do trade deals with non-EU countries post-Brexit.
He added that others are annoyed by the fact that they will have little time to go through the suggestions in depth.
Other critics of the "third way" plan are already calling for the government to maintain its red lines on leaving the single market and customs union.
A group of more than 40 Eurosceptic Tories met Chief Whip Julian Smith on Wednesday to air their concerns about the plan being a soft Brexit which would restrict the UK’s freedom to diverge from EU rules in future.
Jacob Rees-Mogg, head of the pro-Brexit European Research Group told ITV News that Mrs May's plan was simply staying in the single market by the back door: "Regulatory alignment means staying effectively in the single market and the issue there is that trade is more obstructed by non-tariff barriers these days than tariff barriers."
Even Remain-supporting former deputy Prime Minister Nick Clegg spoke out against the plan.
Taking to Twitter, he said Eurosceptic ministers would be "right" to reject it, warning of the risks of increased red tape and "humiliation" for Britain's Parliament.
He added: "Better to put this costly, bureaucratic, unworkable proposal out of its misery ASAP. Whatever Brexit means, it can’t be this."
But Cabinet Office Minister David Lidington accused critics of released "pretty selective leaks" from the Prime Minister's plans.
He said the Prime Minister was proposing a "common rulebook" with the EU on industrial goods and agricultural products "for good practical reasons", such as preventing time-sensitive items like food from being held up at borders.
"This is probably the single most complicated, demanding challenge any government has faced since the Second World War," he said.
"It covers everything from security to scientific cooperation, to universities, to movement of people, to trade investment. So it's a balanced packet is how I think I would describe what's going to be discussed today.
"Yes, we're delivering on sovereignty. Yes, we're delivering on the Prime Minister's red lines. But we are also seeking to do so which looks after jobs, and keeps any disruption to the economy to the bare minimum."
Meanwhile, former Education Secretary Nicky Morgan said any Cabinet minister who did not like the Brexit position agreed at Chequers should resign.
Also speaking to Today, she said a final decision should be reached - and opponents should not seek to alter it.
"What we cannot see is a position that is briefed out tomorrow morning and then over the next 48 hours members of the Cabinet start to undermine or query it or anything else," she said.
"That is what we have seen before when the Prime Minister has made speeches and it's extremely unhelpful.
"Everybody should be able to express their views very thoroughly as to what they think, but when they have come up with an agreed position, if somebody said 'I just cannot live with this, this is not what I want' then yes, I think that they would have to think about their position."
But even if ministers agree on this "third way" at Chequers, it may not be welcomed by the EU.
According to the Financial Times, Brussels will express concern about the reliance of this plan on untested technology and will question how it is not “an open invitation to smugglers”.