A charity founded by Denise Coates, the billionaire boss of online gambling firm Bet365, has reduced its donations to good causes from £9m to £6m, despite its reserves swelling by almost 50% to £568m.

Accounts filed at Companies House detail the charitable activity of the foundation set up by Coates, Britain’s best-paid woman, whose £421m pay-and-dividend package for 2020 took her earnings beyond £1bn in four years.

Divisions of her Stoke-based Bet365 empire donated £100m to the Denise Coates Foundation in the year to 28 March 2021, the accounts show, while investments held by the charity grew in value by £86.5m.

The twin windfalls meant that the reserves of the charity, founded in 2014, soared from £385.3m to £568.4m, a rise of 48%.

The volume of charitable donations declined, however, falling from £9m in 2020 to £6.2m.

The accounts state that donations were “constrained” because the organisations the charity funds have been restricted in their activities. The volume of grants was expected to “bounce back” in future years, the foundation said.

While the reserves of the Denise Coates Foundation increased by £183m, according to the accounts, the charities regulator does not have strict policies on how much should be held relative to donations.

Charity Commission guidance published in 2016 stresses there is “no single level, or even a range of, reserves that is right for all charities”.

According to the guidance, organisations with bigger reserves than necessary could re-examine spending and “trustees must consider whether the purposes of the charity should be amended to enable the charity to operate more effectively”.

The Denise Coates Foundation does address the volume of reserves it holds in comments attached to its accounts.

It states that the charity’s policy is to use the donations it receives from the Coates family business empire to build up its reserves of cash and investments, which rose from £385.3m to £568.3m.

Any income and gains on investments is then distributed over the course of one to five years.

“This policy should allow the foundation to operate on an enduring basis without being dependent on donations from any one source,” the accounts state.

During the year, the charity increased the amount of “unrestricted” funds available for donations by £15.5m to £20.2m.

Donations made in 2021 included £1.9m to University Hospitals of North Midlands, the NHS trust in Stoke-on-Trent, the home of the Coates family, who own Stoke City football club and have based Bet365 there.

In April last year, Denise Coates, who is worth more than £8bn together with her family, announced a £10m donation to the trust, to support staff battling the coronavirus pandemic.

Overall, the foundation made 17 donations to 12 institutions, including a £500,000 grant to the Chronic Disease Research Foundation to research Covid-19.

Other recipients include the Douglas Macmillan Hospice, which received £300,000 to help fund care for 137 patients.

The charity also gave £200,000 towards a Covid-19 appeal launched by Catholic overseas development organisation Cafod and £300,000 towards Kick4Life, a project building a sports stadium in Lesotho to promote social change.

The charity did not make any donations to charities tackling gambling-related harm, either this year or last.

Like other gambling companies, Bet365 donates to gambling-related charities via a voluntary industry-wide levy, which campaigners have said should be mandatory, in order to address the lack of treatment options for gambling addicts in the UK.

The company’s roots are in the network of bookmaking operations started by Peter Coates in Stoke-on-Trent. His daughter Denise is credited with spotting the opportunities offered by taking bets online, building Bet365 into a pioneering and hugely profitable betting firm with global reach.

Bet365 reported an 8% fall in revenue to £2.81bn last year, partly due to the cancellation of sporting events due to the Covid pandemic. Operating profit fell by 74% to £194.7m, down from £758.3m in 2019.

The Guardian has approached a representative of Bet365 for comment.