British troops will return to the front line of the war on terror with a 250-strong unit despatched to “spearhead” the UN’s fight against the world’s fastest-growing Islamist insurgency.

The light-cavalry unit will be flown into Gao in the north of Mali by the middle of this year and will mount 30-day land operations deep into jihadist territory.

The deployment is Britain’s first significant return to an active war zone since the end of Operation Herrick in Afghanistan more than five years ago, a mission that claimed 454 British lives.

The British troops will operate under the UN flag in what experts describe as “the most dangerous peacekeeping mission in the world”. It has already cost the lives of more than 200 UN personnel.

The planned deployment comes amid US warnings that the threat posed by jihadist groups loyal to al-Qaeda and the Islamic State in the Sahel, a vast region of arid scrubland south of the Sahara Desert, could start to threaten Europe as well as the wider region.

The French currently have 4,500 troops in the area but after seven years of fighting they are losing ground.