Christopher Shale, a senior Conservative who was discovered dead in a toilet at Glastonbury on Sunday morning may have died in the early afternoon of the previous day, according to a family friend who said he was briefed by medical staff.

The body of Shale, 56, a close political ally of David Cameron, was discovered on Sunday morning hours after he was quoted in a Conservative strategy document leaked to a Sunday newspaper, as saying the Tories had come across over the years as "graceless, voracious, crass, always on the take" and needed to radically change.

Rupert Soames, a businessman and friend of Shale who was at Glastonbury and has been helping co-ordinate arrangements following the sudden death of his friend said through a spokesman that medics had told him and Shale's family that they believe he died of a massive heart attack "around lunchtime" on Saturday.

Shale, chairman of the West Oxfordshire Conservative Association, had been with his wife and two sons at the Somerset music festival and the family raised the alarm early on Sunday when they realised he had not returned to their accomodation, according to Neil Bennett, a spokesman for Soames.

Shale was found dead in a portable toilet at around 9am and the medics who attended the scene estimated that he was dead inside the cubicle throughout most of the afternoon and through the night, Bennett said.

"Christopher was a dear friend and we are all very much in shock," said Soames, whose company Aggreko supplies the power to Glastonbury.

Michael Eavis, the festival organiser had earlier told a press conference: "We're told it is a suicide situation. It is very, very sad."

But later a police source indicated that ongoing investigations suggested the death was not suicide and Barry Norton, Cameron's electoral agent in West Oxfordshire, said any suggestions that Shale did not die of a heart attack were "scurrilous".

He said he spoke on the phone to Shale at lunchtime on Saturday and they had talked about the article that was about to be published containing his criticisms of the party. "He was very circumspect about it and he was dealing with it and was quite confident it was something that was not really an issue," said Norton.

He added that Shale was "absolutely in good health" but there was a history of heart problems in his family.

Shale's body was taken to a local mortuary in a white private ambulance on Sunday afternoon. Police said a postmortem would take place in due course.

Cameron said he was "devastated" by his friend's death.The prime minister, whose Witney constituency is in west Oxfordshire, released a statement in which he said: "Sam and I were devastated to hear the news about Christopher. He was a great friend and has been a huge support over the last decade in west Oxfordshire.

"A big rock in my life has suddenly been rolled away. Christopher was one of the most truly generous people I've ever met – he was always giving to others, his time, his help, his enthusiasm and above all his love of life.

"It was in that spirit that he made a massive contribution to the Conservative party, both locally and nationally. Our love and prayers are with Nikki and the family. They have lost an amazing dad, west Oxfordshire has lost a big and wonderful man and, like so many others, Sam and I have lost a close and valued friend."

Shale's death coincided with the publication of the article in the Mail on Sunday which revealed the contents of a document, said to be written by him, arguing that the local party needed to change to boost membership, using the codename Operation Vanguard.

Inspector Chris Morgan, of Avon and Somerset police, said: "At 9am this morning, a male has been found down by the press office in the toilet area. At the moment, we're working to establish the cause of the gentleman's death."

Eavis said teams had been out looking for Shale "through the night". Officers were seen comforting a woman after his body was found.

An area between the Pyramid stage and the Other stage was cordoned off on Sunday morning, but reopened by the afternoon with only a minor police presence.

The memorandum, reported by the Mail on Sunday, said Cameron's own association had gained only 22 members in the past year, and Shale was reported to have proposed "a transformational increase" in membership in ways that others could follow nationally.

According to the paper, he wrote that "collectively we are not always an appealing proposition".

He criticised the association's fundraising efforts, saying: "Over the years, we have come across as graceless, voracious, crass, always on the take" and concluded that people did not join because they "think we'll beg and steal from them. And they're right".

Shale added: "When we are together, we are not always a group of people to whom many of our potential members are going to be magnetically drawn.

"When we come together as a group we sometimes morph into something different, less attractive. Our [Woca] environment alters us."

The solution he offered was: "We must look different – when we communicate, when we're together. We must sound different – in what we say, how we say it, the language we use, our tone of voice. We must behave differently – try to see ourselves as others see us."

He claimed the country could be divided into two groups, "politics-heavy" people and "politics-light" ones who are not interested in the subject except at general elections.

He calculated that 98% of the population was "politics light" and that "politics heavy is a big turnoff for politics-light people".

Shale likened changing the membership package away from political activity to what Cameron did to the national party: "It's what, pre-2005, DC used to call double ham and eggs. We've offered them ham and eggs repeatedly. They don't want it. So how can the solution possibly be double ham and eggs?"

Instead, under Shale's strategy, "Woca is, in effect, going into the event management business". He proposed the association put on events with "money-can't-buy appeal".

One idea was for "the PMQ DIY lunch: bring your own sandwiches to watch PMQs in a different fine country house in the constituency (by courtesy of a PPC member) every week; glass of wine, cup of coffee, informal discussion, yours for a fiver".

He also proposed party supporters be given access to politicians in the US in return for cash. "We might have 'Just Another Ordinary Day: we'll organise it but choose how you get there, stay where you like for as long as you like and on one of the days breakfast briefing with a senior staffer, tour of the White House, lunch with a senator ... yours for cost plus a £1,000 donation to Woca."

Shale was a successful businessman as the chief executive of Oxford Resources Ltd, a corporate cost-reduction company based in Chipping Norton. Previously, he was the chief executive of SGL Communications.

He was also a director of the Centre for Policy Studies and a sponsor of OpenEurope, the eurosceptic thinktank.

He was a donor to William Hague's office in opposition, and went to Rwanda with a social action team organised by the Conservatives. He was seen as a modernising rightwinger.