AN investigation was under way last night after four competitors died during the 25th Great North Run.

It is the highest number of deaths in the history of the run.

Ambulance officials said the only thing they could put the deaths down to was the "warm weather and sunshine", with medical staff reporting more casualties than in previous years. It brings the total death toll since the Great North Run began to 12.

Police confirmed that four men died tackling the course, from Newcastle to South Shields, but said the deaths were unrelated.

The hotter conditions are believed to have taken their toll on fun runners, leading to an increase in the casualty figure as a field up of up to 50,000 set out on the 13.1 mile route of Britain's biggest race.

Many competitors found the going tough in the stifling weather, believed to be the warmest since the event was switched to an autumn slot.

Police said the men who died were treated at the scene before being taken to South Tyneside General Hospital, where their deaths were confirmed.

The North-East Ambulance Service, the British Red Cross and St John Ambulance all provided medical assistance for the event.

A spokesman for the ambulance service said: "We all dealt with a higher number of cases at this year's Great North.

"There was a busy period for a couple of hours this afternoon, but overall we coped and the only thing we can put it down to is the sun and warm conditions. Four fatalities in one race is unusual."

He added: "The causes of death will have to be established through post-mortems."

Last year, the run claimed its eighth victim after a 35-year-old man died a mile from the finish.

One of the runners in this year's event, Paul Bouch, 25, said some competitors looked unwell along the course.

He said: "After about ten miles, the ambulances were everywhere and some of the people looked in a bad way.

"But it was difficult to get the ambulances to the roadside because it's just so packed with people."

Mr Bouch, who is studying in Newcastle, completed the course in two hours 10 minutes - 21 minutes slower than last year.

"This year, it was just so much harder," he said.

"I'm just dehydrated and have got a headache more than anything. There's plenty of water along the way but you just try to shut the pain out and carry on. You're not thinking about water during the race.

"But the people on the side of the road were helping out on the way, pointing the ambulances to where they needed to be."

One man was reported to have collapsed near the 11-mile mark and received treatment at the scene.

Event marketing manager Dave Hart, of organisers Nova International, said: "We obviously deeply regret any tragic loss of life at any sporting event and we can only offer our deepest sympathy with everyone concerned.

"We can't comment further as we have still to receive clarification over next of kin being informed. It's a desperately unfortunate situation. There have been extremely warm conditions."

Northumbria police last night declined to reveal further details about the dead men, saying that all relatives had yet to be traced.

A spokesman for the PA Weather Centre said temperatures reached 18C (64F) during yesterday's race and the weather was dry and sunny, with patchy clouds.

It marred an otherwise memorable day for the thousands of competitors who did make it to the finishing line.

Eritrean Zersenay Tadesse took the honours in the men's race, finishing in 59 minutes and five seconds, a second quicker than the previous world record for the half-marathon.