Witnesses claimed the men were killed by firing squad after being tied to poles in the central square of Jilib, a small town in the south of the country controlled by the jihadists.
They were said to have admitted espionage before a court, sentenced by an unofficial judge, then executed.
Recruited Foreign Office staff were yesterday investigating the claims, which were announced by al Shabaab’s regional governor Mohammed Abu Abdalla.
He said: “Five of them were publicly shot to death this afternoon after they admitted espionage before the court.
“Awale Ahmed Mohamed, 32, spied for MI6 and he came from Britain to Somalia to establish Islamic State.”
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Al Shabaab has recruited up to 100 British fanatics during a bloody 12-year war with Somalia’s United Nationsbacked government.
Among them was Thomas Evans, 25, from Buckingham, who was killed in an offensive against Kenyan government troops.
Abdalla, who is in charge of the Jubba area, said that some of those executed had helped guide US drones towards al Shabaab targets.
The terrorists are trying to seize control out of the mayhem which has reigned since the last dictatorship fell 27 years ago to establish Wahhabi, a strict form of Islamic law.
African Union peacekeepers forced them out of the capital Mogadishu, 200 miles to the north, seven years ago, but they have regrouped in the southern zone.
As well as hand-to-hand fighting, al Shabaab has carried out deadly terror attacks, including a massive truck bombing which left more than 500 people dead in Mogadishu last year.
The United States has 500 troops in the country to support the government, while Britain maintains an Army team with medical, logistical and engineering supplies. Government forces and US troops have stepped up their offensive recently and killed a jihadist fighter in an air strike at the weekend.