A 26-year-old British man and his girlfriend were detained by counter-terrorism police at the airport as they returned from the war-torn country.
Officers believe they may have been part of the group which held John
Cantlie and his Dutch colleague, Jeroen Oerlemans, for a week in July.
They are also examining whether the man is the captor who told Mr Cantlie that he was an NHS doctor from London who had taken a sabbatical to wage holy war in Syria. Two homes in east London were also being searched yesterday.
Scotland Yard refused to discuss the arrested man's occupation but confirmed that it was looking at whether the couple were part of the kidnap gang. The pair are the first Britons to be arrested on suspicion of planning or carrying out alleged terrorist activity in Syria.
The intelligence services believe as many as 100 Britons may have travelled to Syria to either join jihadi groups or to fight alongside rebel forces attacking
the regime of Bashar al-Assad.
Mr Cantlie and his colleague were kidnapped in July and both suffered gunshot wounds during a failed escape attempt. They were eventually freed with the help of the Free Syrian Army. Later, Mr Cantlie described being treated by a British doctor in the kidnap group after he was shot.
He said the man, who was armed with an AK-47 rifle, was using a labelled NHS medical kit and when asked his name, simply said: "Just call me the doctor - I'm the only one here."
The medic, who spoke with a south London accent, was also said to be among the leaders of a group who were planning to behead "spies".
Mr Cantlie said the man claimed that he had taken two years out from his work as an NHS doctor to wage jihad, or holy war, in Syria.
"He wanted to specialise in trauma medicine when he returned. He said he'd come here 'to help people' - yet he carried a gun at all times and said he was also here for war," he said.
At the time, the General Medical Council said it would investigate the claims.
Mr Cantlie declined to comment on the latest developments last night.
Counter-terrorism officers from the Metropolitan Police were waiting for the couple, who are both aged 26, when they arrived at Heathrow at 8.30pm on Tuesday, having flown in via Egypt.
They were arrested on suspicion of the commission, preparation or instigation of acts of terrorism.
The couple were being questioned at a central London police station last night.
Police said the arrests were part of an investigation into travel to Syria in support of alleged terrorist activity in the country.
In August, Khalid Mahmood, the Labour MP for Birmingham Perry Barr, said a number of people in his constituency were travelling to the Middle Eastern state, and he expected this to rise.
William Hague, the Foreign Secretary, said the British Government was aware of Britons going to Syria to fight.
"That's not something we recommend, and we do not want British people taking part in violent situations anywhere in the world," he said.