The bodies of two climbers were found this morning on Snowdon as the first fatalities of the severe weather were confirmed.

The brothers, from Weston-super-Mare, are believed to have been caught without ice axes or crampons as conditions on the mountain range became treacherous.

Mountain rescue crews found them on a part of the terrain which suggested that they had fallen as far as 300 metres. The men's family had called emergency services at 9pm last night when they had failed to return home.

The deaths are the first reported as a result of the extraordinary cold snap sweeping through Britain, which has led to a host of accidents and injuries up and down the country.

In one incident, a Cyprus Airways flight skidded after landing at Heathrow shortly after 8.30am, and ended up stuck in the grass. No one was injured in the incident – which did not happen on a runway – and passengers were eventually offloaded and bussed to the terminal. "The plane had safely landed and was making its way to the stand and the front wheel went on to the grass area," BAA, the airport operator, said.

The incident contributed to the huge disruption at Britain's biggest airport, which subsequently closed both its runways leading to more than 250 cancellations and "significant delays" to other flights.

The southern runway eventually reopened at 10.30am and flights were beginning to take off and land again, but there were significant delays and more than 650 cancellations. British Airways announced that all its long-haul and short-haul flights from Heathrow had been cancelled until 5pm, with the exception of three services.

Amid a host of accidents in the capital — which was particularly badly affected by the snowfall — London Ambulance Service said that it had received 650 inquiries between midnight and 7am.

Richard Webber, acting director of operations, said that the service was under "severe pressure". "Our priority is to get to those patients with life-threatening illnesses or injuries as quickly as possible," he said.

"Due to current levels of demand, we will, until further notice, only be sending ambulances to patients across the capital with life-threatening illnesses or injuries. We would urge Londoners to use their ambulance service wisely and only call 999 in the event of a life-threatening emergency."

After a series of minor accidents, drivers were delayed on the M11, leading in to London from the east, with roadside recovery services advising drivers not to travel unless absolutely necessary. Massive delays were also reported on the M25 — stretching 32 miles from Watford, in the north, to Reigate, in the south.

Outside of the South East, a teenage boy was treated for hypothermia at a hospital in Redditch this morning after floundering for 15 minutes in a freezing lake in Worcestershire. Paramedics arrived at the Arrow Valley Country Park just after 3am and found that the 17-year-old had managed to haul himself out on the side of the lake.

In Durham, 22 accidents have been reported on roads throughout the county, where snowfall was also heavy, resulting in minor injuries to drivers and passengers. In one incident, an ambulance overturned and hit a lamppost, although it was not carrying any patients at the time and no one was injured.

In Gloucestershire, a stretch of the M5 near Tewkesbury was closed briefly after a car crashed into the central reservation, causing a multi-vehicle pile up.

In Shropshire, a 15-year-old girl had to be cut free by firefighters after the car in which she was travelling collided with another on a patch of ice last night. She and two boys aged 17 and 18 were taken to the Princess Royal Hospital in Telford with minor injuries.