Reports that UK jihadists were involved in a prisoner swap between Islamic State (IS) and Turkey are "credible", Whitehall officials have told the BBC.
The Times alleges that Shabazz Suleman, 18, and Hisham Folkard, 26, were among as many as 180 IS fighters traded for 46 Turkish hostages.
The Turks were taken prisoner from their country's consulate in Mosul, Iraq, in June and released last month.
Officials confirmed Mr Suleman, of High Wycombe, had disappeared in Turkey.
The Foreign Office is providing consular assistance to his family but there has been no confirmation that he was one of the Turkish government-held prisoners.
"We are aware that a British national was reported missing in Turkey in 2014," a spokesman said.
The Times said it was passed a leaked list of those handed over to IS and the two Britons were among the names.
The newspaper alleges the list also includes three French citizens, two Swedes, two Macedonians, one Swiss and one Belgian.
It also suggests the prisoner agreement included IS fighters held in Turkish hospitals and prisons, as well as those in the hands of moderate Syrian rebels.
Mr Suleman had been a student at the Royal Grammar School in High Wycombe.
In a statement dated 3 October, the school said it had been approached by the "relevant authorities" a few weeks ago and it had fully co-operated.
It said Mr Suleman had worked hard to build on his AS grades to achieve a "solid set" of A-levels and had gained a place at university.
The school described him as "a very engaging and amiable student who fitted in well, forming good relationships with his peers and staff".
The statement also said Mr Suleman had travelled to Syria as part of an aid convoy for a Turkish charity last summer.
The Times also spoke to Mr Folkard's father, who is described as a devout Roman Catholic.
He asked to remain anonymous and is quoted as saying he hardly knew either of his two sons as their mother "took them away".
He told the newspaper she had let them go to Yemen to study Islam and at that point he "cut them off completely".
The release of the Turkish hostages hit the headlines at the end of September. There were 49 in total - 46 Turkish citizens and three Iraqis - and included the Turkish consul, diplomats, special forces police and children.
The BBC's security correspondent Frank Gardner said the 49 hostages were a huge "bargaining chip" and IS must have got something in return.
"This does appear to have been a prisoner swap on a massive scale and one which will alarm the UK and US governments, which do not want to see any concessions made to Islamic State.
"British recruits to Islamic State are not as highly valued as North Africans and Chechens, who tend to have more fighting experience, but the number of UK jihadists making their way to Syria has now passed 500 and is showing no signs of slowing," our correspondent said.
Little explanation was given for their release but Prime Minister Ahmet Davutoglu said Turkey's MIT intelligence agency had led the operation.
At the time, Turkish President Recep Tayyip Erdogan did not deny reports there had been a prisoner swap and stressed no ransom had been paid.
The hostage release paved the way for Turkey, which shares a long and vulnerable border with Syria, to play a more active role in the US-led fight against IS.
Last week, its parliament backed a motion allowing its forces to join the coalition in Syria and Iraq.
The UK joined the coalition of more than 40 nations after MPs voted last month to participate in air strikes.
The latest strike occurred on Sunday night when RAF jets used precision-guided bombs to attack IS fighters at a fortified building near Ramadi, west of the capital Baghdad.
IS - also known as Isis or Isil - controls large swathes of Syria and Iraq after rapid advances through the region this year.
Since August, IS has filmed and posted online videos purporting to show the beheading of four Western hostages.
They were US journalists James Foley and Steven Sotloff and British aid workers David Haines and Alan Henning.
A militant with a British accent has featured in the IS videos.
Security authorities believe more than 500 British nationals have travelled to Iraq and Syria to fight on behalf of Islamic State and other militant groups.