David Cameron’s family received a six-figure windfall from an investor group largely made up of Conservative donors, following the sale of the luxury stationery company that employs his wife, shortly before the start of his campaign to become the party’s leader.

Smythson was acquired in April 2005 by a consortium of City grandees such as Jonathan Green, the co-founder of the GLG hedge fund, Howard Shore, who set up the stockbroker group Shore Capital, and members of the Fleming banking family. All are Tory funders.

It is calculated that Samantha Cameron received more than £400,000 before tax from the sale. She also received a five-figure sum when the company was sold again in 2009 and continues to work for the group.

Other people buying into Smythson in 2005 included Michael Sherwood, Goldman Sachs’s top European executive. His wife, Melanie, is a member of the Conservatives’ Leader’s Group, which requires a minimum donation of £50,000 a year.

Michael Sherwood, Goldman Sachs co-CEO, photographed in 2006. Photograph: Alamy

The decision by so many Conservative donors to participate in the acquisition raises questions as to whether those involved were aware that Cameron was likely to run as leader of the party after that year’s election, and whether the deal was linked to their support for the party. Those involved deny any connection.

The sale was completed on 30 April, five days before the general election saw the Conservatives under Michael Howard heavily defeated. After completion, Samantha Cameron received new shares in Smythson’s holding company, making her the second largest executive shareholder after the company’s chief executive.

The sale of the business was organised by the boutique bank Cavendish Corporate Finance. Its founder is the Conservative donor Howard Leigh, who was given a seat in the House of Lords in 2013.

Samantha, who married David in 1996, had been working at the company since starting as a window dresser in the same year and had become the creative director by the time of the sale.

Shortly after the sale, Paul Herman, then a partner at Cavendish, told the Telegraph that the price achieved “was significantly higher than we first expected. We originally thought it would fetch about £8m.”

Smythson ultimately sold for £15.8m, increasing the value of Mrs Cameron’s 275 shares. Each of the company’s 10,000 shares were worth £1,589.20, which meant she probably received just over £437,000 before tax following the sale of the company.

Samantha also received stock in the new Smythson holding company while most other shareholders left. She received 280,000 share options in Holdsmyth, Smythson’s new parent company.

A Downing Street spokesman said: “The sale of Smythson happened before the 2005 general election, when Michael Howard was leader of the Conservative party.

“Samantha Cameron was not a board director and was not responsible for the sale. It would be entirely wrong to suggest any wrongdoing or attempts to obtain undue influence.”

The structure of the holding company meant that Samantha Cameron received a second windfall when Smythson was sold again in 2009, to a Luxembourg-based buyer. Cavendish also handled the second sale. The business was sold for a headline price of £18m to a group called Greenwill SA, but calculations suggest she received a smaller sum then.

There was a recent controversy over the decision to base Smythson in a tax haven, given that the prime minister has told large firms to “wake up and smell the coffee” over moving funds offshore.

Since 2010, Samantha Cameron has worked two days a week at Smythson as a creative consultant.

The buyers of Smythson in 2005 were described in a statement by Cavendish as King Street Partners, who later became Venrex, and Kelso Place Asset Management. Behind those two names was a group of city grandees, many of whom ended up owning shares in Smythson directly.

Sherwood, the investment banker, declined to comment when the Guardian asked why he had invested.

Green, the hedge fund owner, has given the Conservatives more than £40,000 and was one of the first people to finance Cameron’s leadership campaign in 2005. Green was not available for comment.

Shore, the stockbroker, has given £34,000 personally while Shore Capital has donated more than £580,000 to the Tories. Shore and Shore Capital did not respond to a request for comment.

The veteran financier Sir Mark Weinberg, who has given the Conservatives more than £150,000, also backed Smythson. Weinberg declined to comment.

Smythson makes stationery and luxury goods. Photograph: Alamy

Kelso Place also donated £10,000 to the Conservatives at around the same time as it was involved in the purchase of Smythson. Sion Kearsey, the managing partner of Kelso Place, told the Guardian: “Kelso Place invests solely with a view to generating a return for its investors, and across its 15-year history has made numerous investments across a number of industry sectors, and has returned a multiple of total funds invested.”

The Fleming family, who have given the Conservatives more £1.5m, also took a holding in Holdsmyth through the RF Trustee company and as individuals. The Flemings are long-term Tory supporters. Robert Fleming, who is a director of RF Trustee, also supported Cameron’s leadership campaign in 2005 with a £10,000 donation. Fleming Family & Partners, the wealth management group part-owned by the family, declined to comment.

Lord Leigh has donated more than £125,000 to the Conservatives since records began in 2001, while Cavendish Corporate has given more than £120,000.

Leigh said that he was not involved in the decision to sell Smythson through Cavendish and that the sale was a competitive process with several prospective buyers involved. He said that he did not know David Cameron at the time of the sale and did not support his leadership bid after the election.

Timeline

2000: Howard Leigh, the co-founder of Cavendish Corporate Finance, becomes a treasurer for the Conservative party.

2001: David Cameron becomes the Conservative MP for Witney.

2002: According to his biographers Francis Elliott and James Hanning, Cameron and the Tory MP Greg Barker discuss the possibility of a Cameron leadership for the first time.

Summer 2003: Barker and fellow MP David Ruffley list the likely loyalties of all Conservative MPs in a leadership battle. After analysis, Barker concludes that a Cameron leadership bid “clearly didn’t appear a done deal” but it was “feasible”.

November 2003: Michael Howard becomes the Conservative leader after Iain Duncan Smith is unseated. Cameron is promoted again, becoming the Conservative party deputy chairman.

June 2004: After another promotion, Cameron is put in charge of policy coordination for the next election. A row breaks out in the Conservative party over the rise of the “Notting Hill Tories”, a group of ambitious MPs centred on Cameron.

1 April 2005: An article appears in Times saying that the Smythson stationery company is up for sale and that the sale is being coordinated by Cavendish. Samantha Cameron is Smythson’s creative director.

30 April: Smythson sells for £15.8m to a group of investors, the majority of whom are Conservative donors. Samantha Cameron’s shares are calculated to be worth over £400,000.

5 May 2005: General election. The Conservatives lose for the third time in a row. Michael Howard announces his resignation, but says he will stay on for several months while a new leader is elected. Cameron is promoted to shadow education secretary.

October 2005: After announcing his candidacy, Cameron makes a passionate speech at the Conservative party conference and becomes the favourite in the leadership campaign.

December 2005: Cameron becomes leader of the opposition.

May 2010: Following the general election, Cameron becomes prime minister.