Screening to counter the Ebola threat has started at Britain's biggest airport as David Cameron admitted the system put in place was far from perfect.

The first passengers arriving at Heathrow Terminal 1 after taking indirect flights from West Africa have been questioned to establish any potential exposure to the disease, and had their temperature taken.

The system is expected to be extended to Gatwick and St Pancras Eurostar station in London over the next week.

But public health and Border Force officials admitted there is no fixed plan to deal with people who have visited affected countries, but decline to give details or have their temperature taken.

The Prime Minister told LBC Radio: "Of course we won't catch everybody, of course screening can never be perfect, but it is worthwhile putting in checks.

Sorious Samura, 51, one of the first to undergo the checks, branded them a "joke" after passport staff only realised he had come from Liberia via Brussels when he told them.

Dr Paul Cosford, director at Public Health England, said passengers who boarded an indirect flight to the UK from Liberia, Guinea or Sierra Leone would be automatically flagged at passport control.

But he declined to say that screening would be "mandatory" for those individuals, likely to number a few hundred a month.

"This is a set-up process," he said. "We will be learning from the experience today and over coming days as to how it is working.

"The principle benefit is about distributing information to people about how to contract, the symptoms to look out for, and who to contact in the event that they do get symptoms when they are in this country."

Dan O'Mahoney, chief of staff at the Border Force, said there was no "script" for dealing with unhelpful passengers.

The move came as a United Nations medical worker died in hospital in Germany after contracting Ebola while working in Liberia.

In Madrid, nurse's assistant Teresa Romero Ramos, the first person to contract the virus in Europe, is doing better, but still in a serious condition. A nurse in Texas, who contracted it from a patient, is also said to be 'doing well,' officials said.

A total of 8,914 cases of Ebola have now been reported in west Africa, including 4,447 deaths, but The World Health Organisation said the spread of the disease may be slowing in some areas.

l A team from the public health protection unit of NHS Greater Glasgow and Clyde attended Glasgow Airport yesterday after fears were raised about the condition of a passenger from an in-bound flight. The person did not show signs of the disease and continued their journey.