The X-Men and His Dark Materials star made an initial payment of £25,000 on Saturday.
This came before the group of doctors shared their plan to buy personal protective equipment (PPE) from a verified supplier, prompting McAvoy to donate a further £250,000.
“It was a complete surprise, we didn’t reach out to any celebrities – the crowdfunding company just pointed out that he had made a donation,” Dr Salaj Masand, medical registrar at William Harvey hospital, Kent, told the Standard.
“We’re all speechless and it’s absolutely humbling that not only him but members of the public trusted us to deliver what we said we would.”
The effort has now raised more than £600,000 in 48 hours, triple the initial aim of £200,000 – which has now been upped to £700,000.
The three doctors behind launched the appeal for more masks, gloves and visors in hospitals after feeling they are “going to war without armour or protection”.
They are working with the government to distribute supplies and offer additional money, after ministers received offers from private companies for PPE at inflated prices.
“Time is of the essence, that’s why we raised the capital ourselves,” Dr Masand told the Standard.
“We felt helpless while self isolating and we heard stories of NHS staff buying masks on the internet because of a shortage in supplies.
“What hit home is without adequate PPE, healthcare staff are not only at risk themselves but also to patients and families that don’t have Covid-19.
“I’ve never seen the morale so low in hospitals, and staff are being treated with contempt, raising concerns about PPE, and some hospitals have reportedly told staff with severe asthma to stop wearing masks to stop panicking people.
“We understand that things change every day, but the staff on my wards are being so boosted by the generosity of this campaign.”
It comes after the chair of the Doctors’ Association, representing the NHS frontline, warned staff could quit if the “widespread lack” of PPE provision is not addressed.
Meanwhile, applications have closed for the Government’s biggest call out for volunteers since the Second World War, after 750,000 – triple the original target – came forward.