The return of pre-departure tests for travellers heading to the UK has been described by the travel industry as a "hammer blow" to the sector.
From 04:00 GMT on Tuesday everyone aged 12 and over will have to take a test a maximum of 48 hours before leaving.
The government said the tightening of the rules was necessary because of an increase in cases of the Omicron variant linked to foreign travel, reports the BBC.
But the Business Travel Association said livelihoods would be "devastated".
It came as the latest figures from the UK Health Security Agency on Saturday showed a further 26 cases of the Omicron variant had been reported across the UK - taking the total so far to 160.
Scientists have raised concerns that the heavily-mutated variant may be more transmissible than the dominant Delta strain and be able to escape immunity from vaccines.
Under the changes, travellers will be required to show proof of a negative PCR or lateral flow test taken no earlier than 48 hours before departure, the government announced in a press release.
Currently, travellers only need to self-isolate until they test negative within two days of arriving into the country.
The government said the change to the rules followed new analysis from the UK Health and Security Agency that indicates the window between infection and infectiousness may be shorter for the Omicron variant.
This increases the effectiveness of pre-departure testing as it is more likely to identify positive cases before travel, the government said.
It was also announced that Nigeria is being added to the red list of countries from Monday.
The only people allowed to enter the UK from red list countries are UK or Irish nationals, or UK residents, and they must quarantine in a hotel for 10 days.
Health Secretary Sajid Javid said the measures were temporary.
'It's already here'
However, travel industry bosses said they had not been given sufficient warning.
Clive Wratten, chief executive of the Business Travel Association, said: "The introduction of pre-departure testing with little warning is a hammer blow to the business travel industry."
"Public safety is a priority, but businesses will fail, travellers will be stranded and livelihoods devastated by the lack of coherent plans from government," he added.
Airlines UK said the change was a "premature" move that would "hit industry and passengers before we see the full data" on the effect of the Omicron variant.
The trade body added: "The red list extension made complete sense - that's what it's there for - but we know from experience that blanket restrictions do not stop the importation of variants.
"It's already here. They've now changed their travel advice twice within a week. It's impossible for anyone to plan."
British Airways' Chairman Sean Doyle said the move was "completely out of step with the rest of the world, with every other country taking a measured approach based on the science".
Travel association Abta said the re-introduction of pre-departure tests would be "a huge blow" to "an already devastated travel industry".
The group called for the government "to step up to save jobs and businesses" and for the cost of PCR tests to be reduced.
The Department of Health and Social Care said 21 Omicron cases linked to Nigerian travel had been discovered in recent days.
Mr Javid said this number was growing and Nigeria was now second only to South Africa in terms of linked cases to Omicron.
The Scottish and Welsh governments have confirmed they will also bring in the updated measures for travellers following the assessment of the UK Health Security Agency.
Last week the government's scientific advisory body Sage said pre-departure tests for those arriving in the UK would be valuable, in the leaked minutes of a meeting seen by the BBC.