Travellers to North America face a further wait as US airspace remained closed this evening. .
A midday (5pm GMT) US target for reopening passed without any further provisional deadline being announced. Some carriers have cancelled all flights today, although United Airlines were hoping to resume domestic flights this evening.
Transatlantic flights are unlikely to leave Britain until Friday, according to the Association of British Travel Agents.
Israel has also closed its airports to all foreign carriers, while British Airways has cancelled flights to Islamabad.
The government also announced that passenger planes will not fly over central London , a measure which will add to flight times and increase delays. All private flights were halted except where specifically authorised. City of London airport has been closed, and business aviation flights at Stansted have been banned until further notice.
BA said it had cancelled all of its schedule services between the UK, the US and Canada today as well as suspending flights to the Israeli and Pakistani cities.
"During the course of the day, British Airways will continue to assess the resumption of its services to and from North America," the airline added.
Airlines issued phone numbers for passengers whose flights to the US have been cancelled, or whose flights to other destinations could be delayed.
American Airlines advised British passengers to contact the company on 08457 789 789. Continental Airlines said all their passengers should call 0800 776 464 before going to the airport. The number for British Airways passengers was 08457 799 977.
Security measures had been dramatically stepped up at Heathrow airport with extra police officers being drafted in from surrounding areas and forces.
Armed police patrols have been increased, while passengers boarding flights were also being manually frisked by security guards.
A spokesman for Heathrow said passengers on all routes could expect some delays.
There were few delays at Gatwick airport, which was said to be very quiet with all American services cancelled. Gatwick would normally have handled 18 flights arriving from the US and 14 departing for America, involving around 14,000 passengers.
At Manchester airport, extra security arrangements meant delays of up to an hour and a half, but flights were getting away. Passengers were asked to keep their hand luggage to a minimum.
UK low-cost carrier easyJet today cancelled several flights to avoid congestion resulting from the extra security measures.
The airline advised people to contact its call centre on 0870 6000 000 to arrange a free transfer or refund.
