The 40-year-old suspect arrived in the UK on a flight from Italy on Thursday afternoon.
The Independent understands she is suspected of being an Islamist extremist.
A spokesperson for Counter Terrorism Policing South East said she was stopped by police officers under schedule 7 of the Terrorism Act 2000.
“She was subsequently arrested under Section 41 of Terrorism Act 2000 on suspicion of commission, preparation or instigation of acts of terrorism,” a statement added.
“She was detained under TACT and currently remains in police custody. Enquiries continue.”
The offence of preparing acts of terrorism is used for attack plots as well as travelling to join Isis or other terrorist groups abroad.
Jihadis attempting to return to their home countries in Europe from Iraq and Syria frequently take circuitous routes, with multiple stops, in an attempt to avoid the attention of international law enforcement agencies.
Europol’s annual Terrorism Situation and Trend Report, released last week, said the number of foreign fighters returning to the EU was “very low” as hundreds of European men, women and children remain imprisoned in Iraq and Syria.
“Sweden pointed out that almost all their returnees travelled back using their own travel documents,” the report added.
“Their returnees have not been connected to a confirmed plot and most have returned to their former cities, neighbourhoods and groups of friends, and have taken up activities they were involved in before travelling.”
Europol said an estimated 45 per cent of the 900 Islamists who travelled from the UK to Syria and Iraq have returned to Britain, where only one in 10 have been prosecuted.
“Their training and experience – such as handling weapons and explosives – makes them exceptionally dangerous,” it warned.
“EU member states’ ongoing concern is that these individuals can perpetuate and strengthen their networks both nationally and internationally.”
Europol named Turkey as a key hub for travel, as well as the Western Balkans, Bulgaria, Hungary and Romania.
It said Italy had reported Isis supporters crossing the Mediterranean from Libya, but said that while terrorist use of refugee flows had been observed, it was “not deemed systematic”.