Heathrow Airport has been closed to all incoming flights that are not already in the air, following a police anti-terror operation. A plot has been uncovered to blow up planes in flight from the UK to the US say Scotland Yard.

Terrorists, aiming to commit “mass murder on an unimaginable scale” have been disrupted, according to Scotland Yard. They had planned to detonate explosive devices smuggled in hand luggage on to as many as 10 aircraft.

Police were searching premises with 21 people in custody after arrests in the London area and West Midlands.

The threat is causing delays at all UK airports, with the terrorist threat level to the UK being raised by MI5 to critical. According to MI5's website, “critical threat level”, which is the highest level of threat, means “an attack is expected imminently and indicates an extremely high level of threat to the UK”.