The UK expects to be asked to send troops, with some suggestions up to 1,000 could be deployed as well as a small number of Special Forces.
Their main roles would be to train local forces to oppose IS, which has gained a foothold in Libya since the Western-led intervention in 2011, and to help in operations against people traffickers.
“We have always certainly been clear that if any troops were ever deployed on the back of a request from that government, that they certainly would not be in a combat role,” he added.
Prime Minister David Cameron, who committed British fighter jets to the campaign to overthrow Colonel Muammar Gaddafi in 2011, aired his approval of the deal between the rival governments struck in Tripoli and Tobruk.
“Importantly, this agreement means the international community can now engage with one unified, representative government in Libya in the fight against Daesh [IS] and the migrant traffickers,” he said on Thursday.
In a speech after the deal was struck, UN Libya envoy Martin Kobler said: “The signing of the Libyan political agreement is the first step on the path of building a democratic Libyan state based on the principles of human rights and the rule of law.”
Western Special Forces have not been well received in Libya recently. A group of US commandos was recently ejected shortly after arriving, according to reports.
According to a statement by the Libyan air force, the soldiers, who were heavily armed but in civilian clothes, landed at Wattayah airbase near Tripoli at 06:00 GMT on Monday.
“With the concurrence of Libyan officials, US military personnel traveled to Libya on 14 December to engage in a dialogue with representatives of the Libyan National Army.
“While in Libya, members of a local militia demanded that the US personnel depart. In an effort to avoid conflict, they did leave, without incident,” the Pentagon added.