JustGiving is under pressure to reveal whether it has profited from the £15m raised by Captain Tom Moore after his “100th birthday walk for the NHS” broke the platform’s record for the largest sum raised through a single campaign.

The 99-year-old war veteran completed his goal of walking 100 laps of his Bedfordshire garden on Thursday. He had hoped to raise just £1,000 to support NHS staff through NHS Charities Together, via the popular site JustGiving.

Responding as donations exploded, Neil Coyle, the Labour MP for Bermondsey and Old Southwark, tweeted: “This is amazing. Now we need to make sure @JustGiving do not take a profit off the money intended for the #NHS and those affected by this international emergency. Public donations should reach the intended beneficiary.”

Concerns were also voiced from donors on social media that it was profiting from the colossal fundraising response amid the coronavirus pandemic, although charities praised the platform for being able to withstand a high volume of traffic despite some issues with the site crashing.

JustGiving sought to clarify its business model. The site scrapped its 5% platform fee in 2019, meaning it no longer takes a cut from donations, although it still charges card payment processing fees and takes money from Gift Aid.

It instead moved to a voluntary contribution model, set at a default 10% of the donation amount, which donors have to opt out of by selecting “other” in a list of drop-down options.

The site, which was bought by the US software company Blackbaud in 2017, was criticised for the platform fee it previously charged, particularly in the wake of high-profile appeals such as those for the Manchester bombing victims.

The voluntary contributions, which can be up to 15% of the donation amount, have saved charities £20m in the last year, according to JustGiving.

“Adding a small contribution on top of your donation means we can continue to help more people,” the appeal reads, with a pop-up information bar clarifying that “it will be used to maintain the technology that keeps our site running 24/7” and “provide top-notch customer service”.

Play Video 8:07 How coronavirus changed the world in three months – video

However, in December last year, the US financial consultancy Off Wall Street accused the platform of using “misleading language” by suggesting the money would be reinvested to help charities, “as if JustGiving were a non-profit entity”.

Off Wall Street estimated the new voluntary model earned JustGiving and Blackbaud £7.5m per quarter, £6m more a year than when it charged a 5% platform fee, The Times reported.

If donors contributing to half of Moore’s total paid the default 10% of the donation amount, and those contributing to the other half paid nothing, JustGiving stood to make £750,000.

Blackbaud announced that it would be adding £100,000 to Moore’s fundraiser, the largest donation JustGiving has ever made to a cause.

Payments processed through JustGiving for a UK charity via debit and credit card, and via PayPal, are subject to standard card processing fees of 1.9% plus 20p, and for donations made in foreign currency, this rises to 2.9% + 20p.

If a donation is eligible for Gift Aid, JustGiving said it claims the Gift Aid from HMRC and deducts a 5% processing fee before passing the remaining amount to the charity.

In contrast, Virgin Money Giving, another popular online platform for donating to charity, charges a 2% platform fee and 2.5% payment processing fee, which the donor can choose to cover. If not, the charity will pick up the tab.

JustGiving also makes money by charging charities a membership fee for raising large sums of money through the site: £15 a month for charities raising up to £15,000 a year, and £39 a month for charities raising more than that.

Despite some donors raising concerns on social media and calling on the platform to waive its payment processing fees for Moore’s fundraiser, charities praised the service.

Daniel Fluskey, the head of policy at the Institute of Fundraising, said: “JustGiving has revolutionised fundraising, bringing a new level of sophistication and reliability and while it is a tech company and not a charity it is important we see it as a partner with charities.”

Fluskey said the high volumes of people donating to Moore’s campaign showed how important a reliable platform is. “If you didn’t have that technical sophistication, it’s entirely possible the system would have crashed – and you wouldn’t have raised all that money for the NHS,” he said.

“The experience of our members is that having a good site that they can rely on, that they know works and minimises fraud is incredibly useful, and it is also important that donors and supporters get a smooth experience. If it is clunky and doesn’t work well, people won’t give.”