The British expert in charge of the United Nations response to Ebola has said he hopes the spread of the killer virus will be "under control" in three months.

Dr David Nabarro, the UN's special envoy on Ebola, said the number of cases in west African countries including Sierra Leone were increasing week-on-week.

But systems now put in place, along with international help from nations including Britain, could help turn the tide and reduce the number of cases from one week to the next, he told the BBC 5live Up All Night's Dotun Adebayo.

Dr Nabarro said: "As a result of the sensitisation programme last month I think we have got a much better community involvement, which leads me to believe that getting it under control within the next three months is a reasonable target.

"When we have a disease outbreak like this there is a thing called the epidemic curve, which is numbers of cases over time, and at the moment that epidemic curve is increasing in an exponential fashion ... this means an upward-going curve and it's quite frightening because it means an acceleration.

"By under control I mean we bend the curve down, the numbers of cases each week diminishes compared with the previous week to the point where there is no new transmissions. For me under control means the epidemic curve is coming down and we are confident that the numbers of cases is reducing and will be eventually ended."

Dr Nabarro said people in the affected countries, which also include Liberia and Guinea, now had a better understanding of the need to stop the virus spreading by isolating themselves if they became infected.

He added: "The epidemiologists, the people who study disease, say that when 70% of those who have got the virus are enabled to avoid spreading it to others then the epidemic curve will come down and that's our target for the next 90 days."

His comments come after the UK's chief medical officer, Dame Sally Davies, said the country should expect a "handful" of Ebola cases in the coming months and a major exercise to test the country's readiness for such cases proved plans were "robust", according to Health Secretary Jeremy Hunt.

Enhanced screening for the virus at major airports and terminals, have also formed part of David Cameron's contingency plan against Ebola, which has so far killed more than 4,000 people.

The Prime Minister has been forced to defend the decision after questions were 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.

Dr Nabarro told the BBC screening people arriving at ports and airports was "less necessary" than screening on departure and singling out people because of their nationality or where they are travelling from was "inappropriate".

Boris Johnson said airport screening was a "far from perfect solution" and there would be a case of the disease in London.

The Mayor of London told BBC 1's Andrew Marr Show: "It's one of those cases where we are at risk of seeming to promise stuff that doesn't really make any sense.

"You can't blood test everybody coming into the country."

He added: "The idea of screening it at airports is far from perfect as a solution."

Mr Johnson said there had been "fantastic preparations" to deal with the disease but he expected there to be a case in London.

He said: "I have no doubt, I have little doubt that eventually there will be a case of Ebola in this country and probably in this city."