The UK will require all inbound travellers to take a pre-departure Covid-19 test, and arrivals from Nigeria will have to quarantine in hotels to slow the spread of the Omicron variant, British Health Secretary Sajid Javid has said.
The new rule on pre-departure testing will not apply to passengers arriving from Ireland.
People travelling from within the UK or from Ireland, the Channel Islands or the Isle of Man do not have to fill out a passenger locator form, take any Covid tests or quarantine on arrival in England.
But these exceptions only apply to those who have not been outside the Common Travel Area in the previous ten days.
A spokesperson for the Department of Foreign Affairs earlier said: "We are in contact with the UK authorities to seek clarification on the position of those arriving from within the CTA (Common Travel Area)."
Prime Minister Boris Johnson has said that travel restrictions are necessary to slow the spread of Omicron while scientists race to understand more about the coronavirus variant, its transmissibility and implications for vaccine effectiveness.
"We've kept the data under review over the last week or so since we learned about Omicron, and we're seeing increasing numbers of cases linked to travel," Mr Javid said in a broadcast clip.
"We've always said we will act swiftly if we need to, if the changing data requires that, and that's why we decided to bring in this change on pre-departure tests."
From 4am on Tuesday, anyone travelling to the UK from countries not on the red list will be required to take a pre-departure test a maximum of 48 hours before leaving, regardless of their vaccination status.
And Nigeria will be added to Britain's travel "red list" at 4am on Monday.
Mr Javid said that the number of cases of Omicron in the UK had risen to around 160, and that Nigeria was second only to South Africa in terms of Omicron cases linked to travel.
Under the new rules, from 4am on Monday only British and Irish nationals or residents travelling from Nigeria will be allowed into the UK and must isolate in a government-managed quarantine hotel.
South Africa and nine other Southern African countries were added to the UK red list last weekend.