More than 50,000 people took part in Sunday's Great North Run

Naomi Bradbourne said Reuben Wilson, of Leeds, died despite attempts to resuscitate him by a passing doctor.

She also said runners appeared to be running out of water in the heat.

But run organiser, Nova International, said the men who collapsed on the route received "full" medical treatment within two minutes.

Spokesman David Hart said: "Full medical facilities were at the scene within two minutes in all four cases.

Reuben was so fit - he ran, cycled, and snowboarded. We just don't know what happened Naomi Bradbourne, dead runner's sister

"The people who collapsed on the course were treated with all the facilities that you would have on an ambulance."

The firm also said 16 ambulances were on standby during the event as well as two paramedic bikes and two field hospitals at the finish line

Race chairman, Brendan Foster, has also promised a full inquiry, and said there were 280,000 bottles of water on the route to help combat dehydration.

More than 50,000 runners took part in the 25th half-marathon event, from Newcastle to South Shields, in warm temperatures on Sunday.

The North East Ambulance Service said that in some cases, it was difficult to gain an exact location for patients in need of help.

It also said at least one ambulance had to wait for permission to go on the route and drive against the flow of the runners.

Deputy head teacher Phil Lewis died during the run

Phil Lewis, 52, who taught at Moorside Community College, Consett, Co Durham, and company director Reuben Wilson - the youngest of the four men to collapse during the famous half-marathon - both died from natural causes.

Post mortem examinations are due to be carried out on the two other men, 34-year-old Kieran Patching, of Chatham, Kent, and David Mahaffey, 43, from York, later today.

Mr Wilson was doing the run to raise money for Martin House Hospice in Leeds, which has helped his three-year-old nephew Miles, who is suffering from a terminal genetic disorder.

Ms Bradbourne, 34, was also taking part in the run.

"I was running out of water after about an hour and saw about 10 people who had fallen along the wayside in the heat. I then spotted my brother in the commotion.

"A doctor in the crowd administered CPR (cardio-pulmonary resuscitation) and never stopped working on him, but it took 45 minutes for the ambulance to arrive and it was the longest time of my life.

'Everyone coped'

"Reuben was so fit - he ran, cycled, and snowboarded. We just don't know what happened," she said.

Mr Foster, chairman and founder of the run, said: "We have been through all procedures with our race doctor and there were dehydration issues, but these were well covered.

"There were 280,000 bottles of water along the route and although the finish line was busy it was not over-run - everyone coped very well.

"We will carry out an inquiry and welcome any other agencies who can help - we have on our hands a complete tragedy."