A Christmas bubble loophole means families could spend at least six days together if "unforeseen disruption to travel" comes into play.
Brits are allowed to mix three households indoors between December 23 and 27 as part of a coronavirus restrictions loosening during the festive period - despite fears of a third wave.
But new regulations published on Monday say the bubble could last longer than the initial five days for some if their train is delayed or their car breaks down.
The Government guidelines say bubbles can be temporarily extended "immediately after the Christmas period", with the loophole applying to all members even if only one person is impacted.
The regulations state: "This subparagraph applies where one or
more persons at the gathering has not been able to return to their home because of unforeseen disruption to travel.
"Where a member of a household is or has been in a linked Christmas household in relation to members of two other households, the person cannot be linked with the members of any other household under this regulation."
Meanwhile, there are exceptions for kids whose parents live separately and students can travel home from Thursday before returning to university up to February 8.
Rule breakers face fines of to £6,400 or could be taken to court.
While businesses could be forced to pay out up to £10,000 for organising illegal gatherings.
From Wednesday, all but a few hundred thousand people in England will be in Tiers 2 or 3, banning mixing doors as Boris Johnson's national lockdown comes to an end.
Tier 2 measures also include a ban on going to the pub unless it serves substantial meals and only with people in your household.
In Tier 3, which consists of around 23.3 million people living in large swathes of the Midlands, North East and North West, pubs and restaurants are closed except for takeaway drinks.
The tier system is set to remain in place until March.
Mr Johnson has emphasised the tiers will be reviewed every two weeks, with the first on December 16.
MPs will vote on the Tiers plan on Tuesday.
The Prime Minister reportedly faces a mutiny from up to 70 Tory MPs after laying out which areas would be subject to which tiers, with only Cornwall, the Isle of Wight and the Isles of Scilly in Tier 1 - amounting to 714,000 people.
Around 16 million people in Tier 3 areas are expected to be downgraded to Tier 2 at the first review.
Chris Whitty, the chief medical officer for England, however, previously put the dampeners on the prospect of widespread easing prior to Christmas, arguing "almost certainly anywhere that goes into Tier 1” will experience a rise in cases".
He said the Government could only risk dropping areas into the bottom tier if they're "very confident indeed", with winter being a notoriously difficult time for the already stretched NHS.
Professor Whitty added that only when vaccines are rolled out next year could the country "little by little" begin to "walk out of this down the tiers".
