Heathrow and all other UK airports put on critical security alert this morning

Security at the construction site at Heathrow Terminal 5 has been ramped up following the disruption of an alleged plot to blow up planes from UK airports mid-flight.

UK airports were this morning put on “critical” security alert by the government after Scotland Yard announced it had uncovered a plot to blow up planes in flight from the UK to the US.

Although workers at Heathrow T5 are being allowed access to the site, BAA has put new security measures in place to prevent the site being used to gain unauthorised access to the airport.

A senior project source said: “The reality is that the current situation will affect the site. BAA will brief the workers this afternoon in terms of the new security arrangements.”

It is understood that terrorists had planned to detonate explosive devices smuggled aboard planes in hand luggage. Police have arrested 21 people in London and the West Midlands in connection with the alleged plot.

According to the MI5 website, a critical threat level means an “attack is expected imminently and indicates an extremely high level of threat to the UK.”

Heathrow Airport has been closed to all incoming flights that are not already in the air. Several outbound services have also been cancelled. High security is causing delays across UK airports.

Metropolitan Police deputy assistant commissioner, Paul Stephenson said the alleged plotters had intended "mass murder on an unimaginable scale".