Travellers heading to the UK will now have to take a Covid test before their departure in an effort to limit spread of the virus, the government has said.
Health Secretary Sajid Javid said the tightened requirements would come into force from 04:00 GMT on Tuesday.
Travellers will be required to submit evidence of a negative lateral flow or PCR test to enter.
Currently people only need to self-isolate until they test negative within two days of arriving.
Nigeria will be added from Monday to the red list of countries from where people arriving must quarantine in a hotel for 10 days, Mr Javid also confirmed.
The health secretary said the government’s strategy since the discover of the Omicron variant had been to “buy time” to assess and to “put in place protective measures”, adding “we’ve always said that we would act swiftly should new data require it”.
He said: “Over the recent days we have learnt of a significant number of growing cases linked to travel with Nigeria.
“There are 27 cases already in England and that’s growing and Nigeria now is second only to South Africa in terms of linked cases to Omicron.”
Mr Javid added the government had “been clear that we will take action if it is necessary”, adding it was important to remember vaccinations “are our first line of defence” and calling on people to get a booster dose of a Covid vaccine when called to do so by the NHS.
The moves come after pressure on the government to tighten the policy had been growing over the course of the week.
The Labour Party previously criticised the government’s current testing policy and called for the reintroduction of pre-departure tests.