David Cameron intervenes as charities admit sending appeals to 92-year-old pensioner Olive Cooke

Leading charities have admitted sending appeals letters to an elderly grandmother who died after receiving very large numbers of requests for donations.

Last night Amnesty International, Save the Children and the Alzheimer's Society insisted that they bore no responsibility for 92-year-old Olive Cooke's death. Prime Minister David Cameron has intervened to call for charity watchdogs to investigate the high volumes of charity letters sent to her.

"Olive Cooke was an incredible woman who worked tirelessly for the charities she supported," said Cameron. "There is a code that is meant to protect people from feeling pressured by charities and I hope the Fundraising Standards Board will look at whether any more could have been done to prevent this."

Cameron awarded Cooke an award last November for being Britain's longest serving poppy seller.

The number of charity appeal letters she received each month is not known, but estimates range between 180-260. Cooke also got large numbers of phone calls from charities. Her body was found in the Avon Gorge last week, after she told her friends and family that "she couldn't take it any more".

Cooke's grandson Kevin King, 38, said that some charities might have been attempting to "milk" her. "When people phoned she ended up feeling quite guilty. She took it to heart," he said. "I think they were pestering her too much. It was like they were trying to milk her."

Cooke's name was on a list of potential donors made available to charities by private firms, which led to her being swamped by requests for money.

Other high profile charities who were writing to Cooke included Battersea Dogs & Cats Home, Prostate Cancer UK and Breast Cancer Care. All the charities maintain that they have adhered to the highest ethical standards.

Kate Allen of Amnesty International said: "I am deeply saddened by the news of Olive's tragic death – our thoughts go out to her family at this time. We are taking this issue very seriously and are looking into the details. Olive was a long-standing and valued supporter of Amnesty. Our team last telephoned Olive in April. During that call, even though Olive did not say so explicitly, we sensed she would prefer not to be called again. We then amended our details immediately. This was the last and only call we made."

The Institute of Fundraising (IoF) Standards Committee said it will bring together representatives from across the charity sector to review the case.

More than a week after Cooke's death on 6 May, charity appeal letters are reportedly still being delivered to her home in Bristol. An inquest into her death is expected to open next week.