Heathrow airport is to start screening for Ebola among passengers flying into the UK from countries at risk.
The health secretary told MPs a "handful" of cases were expected to reach the UK before Christmas.
Screening will start at Terminal 1, before being extended to other terminals, Gatwick airport and Eurostar by the end of the week.
In September, about 1,000 people arrived in the UK from Ebola-affected countries in West Africa.
Meanwhile, a UN medical official infected with Ebola while working in Liberia has died in a hospital in Germany.
Ebola: What to do in the UK?
Symptoms of Ebola include fever, headache, vomiting, diarrhoea, bleeding - but these are similar to more common infections like flu and some stomach bugs.
If you have these symptoms and have had contact with an Ebola patient then ring 111 first, do not go directly to A&E or a GP.
If there has been no contact with Ebola then seek help from 111, your GP or A&E if necessary.
The chances of developing Ebola in the UK remain low.
Most passengers flying from Liberia, Sierra Leone and Guinea, where Ebola has killed more than 4,000 people, are screened before being allowed to board the plane.
Under the new UK screening measures, they will be identified by Border Force officers upon arrival. Nurses and consultants from Public Health England will then carry out the actual screening.
Passengers will have their temperatures taken, complete a risk questionnaire and have contact details recorded.
Anyone with suspected Ebola will be taken to hospital.
Passengers deemed to be at high risk due to contact with Ebola patients, but who are displaying no symptoms, will be contacted daily by Public Health England.
There are no direct flights to the UK from Liberia, Sierra Leone or Guinea, which means people travelling from those countries would have to catch a connecting flight to the UK and could arrive at airports that are not screening passengers.
Instead "highly visible information" will be in place at all entry points to the UK.
The Department of Health estimates that 85% of all arrivals to the UK from affected countries will come through Heathrow.
BBC transport correspondent Richard Westcott said there was one flight on Tuesday which would have been subjected to the new screening measures.
It was the Brussels Airlines flight which left Liberia for Brussels on Monday night, with transfers coming into Heathrow at 09.30 BST on Tuesday.
The screening will be extended to Terminal 2 on Wednesday, and rolled out to the other three terminals by the end of the week, our correspondent added.
"There is currently no screening at Brussels and Paris where most direct flights from the affected area will land or transfer to the UK," he said.
ITV correspondent Neil Connery flew into Heathrow from Brussels on Tuesday morning and said he was asked at border control where his journey had started.
"A small team of border officials wearing blue protective gloves questioned some passengers upon Lhr arrival," he tweeted.
The Independent's travel editor Simon Calder said three airlines were operating out of the three most-affected countries - Air France, Royal Air Maroc and Brussels Airlines - and there were only a handful of flights a week.
Screening arrivals marks a rapid shift in policy from the UK government.
Last week, it said there were no plans for screening as people were tested before leaving affected countries.
The WHO said it was unnecessary and that it would mean screening "huge numbers of low-risk people".
In the Commons on Monday, Health Secretary Jeremy Hunt said the UK needed to prepare for the situation deteriorating in West Africa.
He said: "[The chief medical officer] confirms that the public health risk in the UK remains low and measures currently in place, including exit screening in all three affected countries, offer the correct level of protection.
"However, whilst the response to global health emergencies should always be proportionate, she also advises the government to make preparations for a possible increase in the risk level."
Mr Hunt added that tackling the outbreak in Africa was the "single most important way" of preventing Ebola arriving in the UK.
The UK government has pledged £125m "to help contain, control, treat and defeat Ebola", hundreds of NHS staff have volunteered to travel to West Africa and 750 troops have been deployed to help build treatment centres and provide logistical support.
The UK's casualty vessel RFA Argus, which has a fully-equipped hospital, is expected to set sail for Sierra Leone later this week. It will not be used to treat Ebola-infected patients but mainly to ferry kit and people.
There is no cure or vaccine for Ebola, which is transmitted through sweat, blood and saliva.
Anyone in the UK with suspected Ebola will be taken to hospital and blood samples will be taken to Public Health England's specialist laboratory for rapid testing.
If the test is positive, then the patient will be transferred to an isolation unit at the Royal Free Hospital in London. It is the centre that cared for the British nurse William Pooley, who contracted Ebola in West Africa.
Hospitals in Newcastle, Liverpool and Sheffield are on standby to offer similar facilities if there is a sudden surge in Ebola cases. A total of 26 isolation beds could be prepared at the four hospitals.
Shadow health secretary Andy Burnham told BBC Radio 5 live that introducing screening "was the right thing to do" based on advice from the chief medical officer.
He said there was a shortage of the experimental drug ZMapp, which has been used to treat Ebola patients, and more should be done to expand the manufacturing capacity.