British troops must stay in Afghanistan or else millions of refugees will flee to Europe, the UK defence secretary has said.

Since combat operations ended in 2014, there are now 500 British soldiers who are training local military. However, with plans afoot to increase the number of UK troops in the country, Sir Michael Fallon said that Afghanistan risks collapse if soldiers pull out.

He told the Munich Security Conference: "If it was right to go in, it has to be right not to leave before the job is done as well as we can do it.

"If this country collapses, we here will feel the consequences, very directly. There could be three to four million young Afghan men sent out by their villages to migrate westwards, and they are heading here".

The defence minister, Mike Penning, has said that more soldiers will go to Afghanistan to help further train local troops to tackle the Taliban in the country, which has fallen out of Kabul's control.

At one point there were 10,000 British troops in Afghanistan, with some 456 personnel killed.

As well as the Taliban resurgence, Isis has made gains in the eastern province of Nangarhar and al-Qaida is said to be present in Kandahar province.

"There are trans-national terrorist groups in Afghanistan that wish us harm at home and here in our democracies. That is why we went in in the first place and those transnational terrorist groups are still there and they still pose a threat," Fallon said, according to The Times.

Earlier in February, the US commander Gen John Nicholson asked for several thousand more troops to break the stalemate against the Taliban. He told a Senate panel that the troops were required to train Afghan forces to replace Nato and US troops.