The two people arrested at Heathrow Airport on suspicion of committing terror offences flew into the UK from Egypt, Scotland Yard said.
Police did not make clear whether the suspects were believed to be returning from or heading to Syria.
The pair, who have not been named, were arrested of the "commission, preparation or instigation of acts of terrorism".
More top news
Weather throws a wobbly as it's seriously disappointing for summer.
The Bank of England governor has suggested there may soon be a cut in interest rates.
The motorist at the wheel of a self-driving car has died after it crashed with a trailer, the first Autopilot death reported in the US.