THE number of deaths in the outbreak of Ebola that has gripped West Africa has now passed 4,000, the World Health Organisation (WHO) has said.

Officials confirmed that the death toll stands at 4,033, including 2,316 in Liberia, 930 in Sierra Leone, 778 in Guinea, eight in Nigeria and one in the United States.

The outbreak is now the worst in history and has left governments and health agencies around the world scrambling to bring in contingencies to stop the spread of the disease.

A national exercise to test Britain's readiness for an Ebola outbreak is due to take place today, with Government ministers joining dozens of medical professionals from hospitals, the ambulance service and Public Health England for an eight-hour exercise in locations across the country.

Actors will simulate symptoms of the deadly virus to test the response of emergency services, while some staff will wear personal protective equipment.

A simulated meeting of the Government's Cobra emergency committee will also be held, chaired by Health Secretary Jeremy Hunt.

The exercise was ordered by David Cameron and comes after the Prime Minister was forced to defend the decision to introduce enhanced screening for the virus at major airports and terminals, saying it had been taken on "medical advice".

Questions have been raised about the checks, which are to take place at Heathrow, Gatwick and Eurostar rail terminals, with a spokesman for Gatwick saying that the airport had not been given any instructions about how the screening should be carried out.

The move was also criticised by health experts, with one describing it as a "complete waste of time", while Labour MP Keith Vaz said the lack of precise information available about the screening was "shambolic".

David Mabey, professor of communicable diseases at the London School of Hygiene and Tropical Medicine, said it was only a matter of time before a case was reported in the UK.

He said: "Firstly, there won't be anyone coming from these (West African) countries because all direct flights have been cancelled," he said. "Are they going to screen everyone from Brussels, Paris, Frankfurt and Amsterdam? That would lead to a lot of delays and disruption.

"Secondly, why would anyone admit that they have been in contact with someone when they're in a hurry to get through immigration control when they arrive at the airport?

"People will have been screened before getting on flights and may seem to be healthy if they're not yet showing symptoms.

"And then what will they do if they think they do have Ebola? Lock them up in solitary confinement?"

Prof Mabey said that, instead, the Government should be concentrating on increasing awareness of what people should do if they think they might be at risk.

Asked if he thought Ebola would reach the UK, he said: "I think it's bound to happen. We have a lot of links with West Africa. There is lots of coming and going so I'm sure there will be a few cases."

However, Mr Cameron said: "What we do is listen to the medical advice and we act on that advice, and that's why we are introducing the screening processes at the appropriate ports and airports. What we are focusing on as a country is taking action right across the board to deal with this problem at source."

The Department of Health (DoH) has not revealed the locations of today's exercise. A spokeswoman said: "The public can be assured that we have been planning our response to an Ebola case in the UK for many months now since the outbreak started in West Africa. It is vital that we test these plans in as realistic a situation as possible - with real people.

"That will include a full COBR meeting chaired by Health Secretary Jeremy Hunt to assess the outcome and learn any wider resilience lessons. It is important to remember that the overall risk to the public in the UK continues to be very low, and the UK has some of the best public health protection systems in the world with well-developed and well-tested systems for managing infectious disease."