The UK counter-terrorism officers have arrested a woman at Heathrow Airport on suspicion of plotting an attack.
The 27-year-old UK national had just arrived on a flight from Addis Ababa, the capital of Ethiopia, when she was detained on Friday, Scotland Yard said.
She is being held on suspicion of preparing a terrorist act under section 5 of the Terrorist Act 2006.
Police said they had taken the woman to a south London police station, where she remains in custody.
The current threat to the UK from international terrorism is “severe”. This means that a terrorist attack is “highly likely”.