Theresa May will warn world leaders they must move quickly to stop Isis gaining a foothold in other African countries, in a bid to demonstrate that the UK is still at the forefront of the fight against terrorism.

In a speech she is due to give at the G20 summit in China, she will suggest inaction could allow the terror group’s fighters to establish themselves in sub-Saharan nations.

It will see the Prime Minister sticking to what she knows at her first major international conference, having dealt in detail with the terror threat and extremism as one of the longest serving Home Secretaries.

A UK official said: “She will be warning leaders that as we make progress against Daesh [Isis] in places like Iraq and Syria and Libya and squeeze down the territory they can occupy there, we need to watch for terrorists moving to other countries in that region.”

Mrs May will highlight how action in Libya supported by US air strikes has reduced “the Daesh footfall” from 200km of coast line in April to just one square kilometre in Sirte now.

Libya: ISIL loses a strategic base in Sirte

The official added: “What we are starting to see is some flows of terrorist fighters heading south out of Sirte towards sub-Saharan Africa.”

Mrs May will push world leaders to work together as they have been doing to recapture territory in Syria and Iraq.

The Prime Minister will make tackling the terror threat a key part of her G20 summit after warning that a UK strike was “highly likely” in the wake of recent attacks on mainland Europe.

Terrorism in 2016: Terror attacks in Europe claimed by Isis Show all 9 1 /9 Terrorism in 2016: Terror attacks in Europe claimed by Isis Terrorism in 2016: Terror attacks in Europe claimed by Isis Policemen outside Rouen's cathedral during the funeral of Jacques Hamel, the priest who was killed in a church in Saint-Etienne-du-Rouvray in Normandy on 26 July during a hostage-taking claimed by Islamic State group Joel Saget/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis Two jihadists, both 19, slit Hamel's throat while he was celebrating mass in an attack that shocked France as well as the Catholic Church Joel Saget/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis Muslims place flowers and hold a minute of silence in front of the church if Saint-Etienne-du-Rouvray, western France, where French priest Jacques Hamel was killed on 26 July Charly Triballeau/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis Two people hold each other by the new makeshift memorial in Nice, in tribute to the victims of the deadly Bastille Day attack at the Promenade des Anglais Valery Hache/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis The Islamic State group claimed responsibility for the truck attack that killed 84 people in Nice on France's national holiday. Mohamed Lahouaiej-Bouhlel, 31, smashed a 19-tonne truck into a packed crowd of people in the Riviera city celebrating Bastille Day Valery Hache/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis Police work at a site where a Syrian migrant set off an explosive device in Ansbach, southern Germany, on 25 July, killing himself and wounding a dozen others Daniel Roland/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis A Syrian migrant set off an explosion at a bar in southern Germany that killed himself and wounded a dozen others in the third attack to hit Bavaria in a week. The 27-year-old, who had spent a stint in a psychiatric facility, had intended to target a music festival in the city of Ansbach but was turned away because he did not have a ticket Friebe/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis Police officers walk along train tracks in Wuerzburg southern Germany on 19 July, a day after a man attacked train passengers with an axe. German authorities said they had found a hand-painted IS flag among the belongings of the man, an asylum seeker from Afghanistan, who seriously injured four members of a family of tourists from Hong Kong in his rampage Daniel Roland/AFP/Getty Images Terrorism in 2016: Terror attacks in Europe claimed by Isis German police killed a teenage assailant after he attacked passengers on a train in Wuerzburg, southerg Germany with an axe and a knife on 18 July, seriously wounding three people Karl-Josef Hildenbrand/AFP/Getty Images

The threat from international terrorism has been "severe" since 2014, when it was upgraded from "substantial".