A fit and healthy woman who collapsed near the finish of the London Marathon probably died as a result of taking a legally available performance-enhancing drug, a coroner has ruled.

Claire Squires, 30, collapsed in Birdcage Walk, a mile from the finish line of the 26.2 mile event on April 22 last year.

Coroner Dr Philip Barlow said no one should believe that Miss Squires, a 30-year-old hairdresser from North Kilworth, Leicestershire, regularly took it, but the drug 1,3-dimethylamine (DMAA) was an ingredient in a supplement which she had bought legally online.

Dr Barlow, sitting at Southwark Coroner's Court, said: "Claire Squires collapsed during the final stages of the London Marathon. She had taken a supplement containing DMAA which, on the balance of probabilities, in combination with extreme physical exertion, caused acute cardiac failure, which resulted in her death."

The inquest heard that the "amphetamine-like" drug DMAA, which is found in some sports nutrition supplements, was found in her system.

The coroner said he hoped the case would highlight the potential dangers of DMAA, which increases the heart rate. He said: "My hope is that the coverage of this case and the events leading up to Claire's death will help publicise the potentially harmful effects of DMAA during extreme exertion."

Turning to the family, who had sat and listened quietly throughout the hearing, he said: "She was obviously a very dedicated and well motivated person. She died raising money for charity. I can only offer my condolences to all members of the family for a very tragic loss of an obviously dear person."

Miss Squires, a regular runner who had run the London Marathon two years previously, had aimed to raise £500 for Samaritans but the total jumped to more than £1 million following a public outpouring of support in the aftermath of her death.

She had a scoop of the supplement, called Jack 3D, in her water bottle, according to her boyfriend Simon van Herrewege. Outside the court Mr van Herrewege said: "Claire would never ever have taken anything that would cause her harm or even worse risk her life. She innocently took a supplement which at the time was entirely legal and widely available on the high street and somewhat worryingly apparently used by so many others. It is clear that there needs to be far better supervision of the so-called health foods and supplements industry so that no more tragedies like this happen again causing other families to have to go through what we have been through this past year.

"Claire was a beautiful person committed to helping others. We have been comforted by all those who knew and loved Claire and by the heart-warming support shown by the general public through donations to the Samaritans and subsequent charitable events to raise money for worthwhile causes in her name. The overwhelming response will help Claire's memory live on and hopefully help others."