Counter-terror police have arrested a British man at Heathrow airport after Turkey said it had deported an Islamic State suspect to the UK.

The Metropolitan police’s counter-terrorism command said a 26-year-old British man was arrested on arrival at Heathrow on suspicion of preparation of terrorist acts, adding that the unidentified suspect remained in police custody and the arrest was “Syria-related”.

No other details were disclosed, but Turkey said earlier on Thursday that a Briton was among eight Islamic State suspects it had deported as part of a controversial push to repatriate foreign Isis suspects held in Turkish jails.

Security sources told the Guardian the suspect was not a major Isis figure and may only have travelled to the region recently.

It is not known how many UK nationals are held in Turkey. Only one has been publicly named: Aine Davis, a 35-year-old member of Isis’s “Beatles” cell which beheaded hostages in Syria, is currently serving seven and a half years in a Turkish jail.

A second man, 23-year-old Shabazz Suleman, who was freed from Syrian rebel detention in February, had expressed a desire to travel to Turkey after a local opposition-run court cleared him of all charges. His current whereabouts are unknown.

British diplomats said earlier this week they were not expecting any detainees from Turkey, and British politicians were caught unawares on Thursday.

Initially it had been thought that both British police and politicians did not know about the deportation, but that was subsequently corrected by security sources who said while police were aware and waiting at Heathrow airport in advance, politicians had only become aware at the last minute while the individual was on the plane to the UK. “This was a managed return, but not everybody knew about it,” a security source said.

The Home Office said it did not comment on individual cases, although its standing policy is that any Briton who returns to the UK from having taken part in the conflict in Syria or Iraq must expect to be investigated by police and could be prosecuted.

About 1,200 foreigners are being held in detention centres in Turkey along with 287 more captured recently in Turkey’s invasion of Kurdish-held parts of Syria last month.

About 900 Britons are believed to have gone out to fight for Isis in Syria and Iraq. Half of them have already come back to the UK, with some remaining under active monitoring since their return. The remainder have either been killed, are in detention or remain free in the region.

Ankara has long voiced frustration with its European allies for refusing to take their citizens home. However, Turkish officials have expressed a new-found determination to deport foreign jihadists after facing international condemnation over the attack on western-backed Kurdish-led forces over the border in Syria, which Ankara considers a terror group.

Speaking to reporters earlier this week, the Turkish president, Recep Tayyip Erdoğan, threatened to release all of its jailed foreign militants and send them to Europe.

“You should revise your stance towards Turkey, which at the moment holds so many Isis members in prison and at the same time controls those in Syria,” Erdoğan said, addressing European countries.

“These gates will open and these Isis members who have started to be sent to you will continue to be sent. Then you can take care of your own problem.”

One UK citizen and at least seven Germans, as well as French and Danish nationals are believed to have been deported so far, despite widespread confusion in diplomatic circles over how the new policy will work.

One alleged Isis member, a US man of Jordanian descent, was left stranded in the no-man’s-land between the Turkish and Greek borders for three days this week after Athens refused him entry.

The man was given food by Turkish border guards and allowed to sleep in a car before the Turkish interior ministry said on Thursday he had been taken back into custody after the US agreed to receive him.

Turkey has not given total numbers and nationalities of those it is planning to send home. The policy is particularly complicated when it comes to stateless individuals – many of whom have been stripped of western citizenship in an attempt to prevent them returning home.

Several countries request passenger lists for both military and commercial flights before a plane is allowed to enter their airspace.

• This article was amended on 14 November 2019. An earlier version said police had been unaware of the suspect’s imminent arrival. This reference has been changed.