The web site for the British security agency MI5was also unavailable at times this morning amid heavy traffic. The MI5 site displays the current UK terror threat level, which was raised to “critical” this morning after British authorities said they thwarted a terrorist plot to simultaneously blow up several aircraft heading to the United States using explosives smuggled in carry-on luggage.

“Due to the heightened security at UK airports, the BAA website is currently experiencing a high level of people visiting the site,” the company said in a statement on the revamped baa.co.uk site. “All BAA airports remain open but passengers should expect severe delays at Heathrow, Gatwick and Stansted.”

BAA said today it had requested that all in-bound flights from Europe into Heathrow be cancelled temporarily. “We would also advise those passengers who must travel today to consider using public transport and be prepared for long delays as a result of (additional security) measures.”