The first ever excavation of the main allied field hospital at the Battle of Waterloo has uncovered sawn-off limbs and musket balls fired during a previously unrecorded fight on the steps of the farm where the Duke of Wellington’s medics worked.

The surprise find by British and Dutch archeologists, digging alongside 25 military veterans, opens up a new understanding of how the Mont-St-Jean field hospital was engulfed by war on 18 June 1815.

The pioneering archaeological survey has been expanded as a result of the discoveries, which includes 58 musket balls unearthed this Monday alone.

About 6,000 wounded men are believed to have passed through the makeshift hospital during the Battle of Waterloo to receive primitive medical care for their injuries, including amputations without anaesthetic.

Among the patients there were William II of the Netherlands, otherwise known as the Prince of Orange, and Wellington’s military secretary, Lt Col Lord FitzRoy Somerset, who had his arm amputated.

Archaeologists found three leg bones at the site, one of which bears the marks of the surgeon’s saw from an amputation above the knee, providing gruesome evidence of the surgeons’ work.

Facebook Twitter Pinterest The Waterloo Uncovered team in the courtyard of Mont-Saint-Jean farm. Photograph: Matt Weston

The heavy concentration of balls found at the site, fired both from the allied Brown Bess infantry muskets and the smaller calibre French weapons, suggests that a fight also took place there, potentially after Napoleon’s generals ordered a cavalry charge on the grounds of Mont-St-Jean, now an orchard, which lay about 0.3 miles (600 metres) behind the main allied line.

A six-pound cast iron French cannon ball found on the site is believed to have been fired late in the day of the battle, at about 6pm when French troops captured the farm of La Haye Sainte.

At that point the French were closest they would ever get to victory and able to bring up horse artillery batteries of the Imperial Guard to bombard the allied lines with round shot and canister from very close range, causing devastating casualties. It was only the arrival of Prussians on the extreme left flank of Wellington’s army that saved the front from being broken.

Prof Tony Pollard from the University of Glasgow, who is the lead academic on the Waterloo Uncovered dig organised by the Waterloo Uncovered charity, said the discovery of human remains was unexpected and poignant.

He said: “In one of the trenches surveyed by metal detectors. a signal relating to a large metal object led the team of archaeologists to excavate further. They found human remains, the first time Waterloo Uncovered has encountered such a find.

“After working with the local authorities to establish that the bones did not relate to a modern burial, work continued and has revealed at least three leg bones. These appear to be the remains of amputated limbs from some of the operations carried out by surgeons.

“Finding human remains immediately changes the atmosphere on a dig. Suddenly there is a very poignant connection with the people who suffered here in 1815, a connection that has not been lost on the Waterloo Uncovered team of veterans and serving personnel. The next stage is to carefully excavate and remove the bones for further examination.”

Facebook Twitter Pinterest A group photo of the 2019 Waterloo Uncovered team at Mont-Saint-Jean. Photograph: Chris van Houts

Waterloo Uncovered, a charity bringing archaeologists together to work with ex-military personnel, was founded by two Coldstream Guards officers, Charles Foinette and Mark Evans, who suffered from post-traumatic stress disorder (PTSD) after a tour in Afghanistan.

Among the group working on the site are military personnel wounded or suffering from PTSD after action in Iraq and Afghanistan. Their ages range from 19 – a soldier serving in the Coldstream Guards who is recuperating from training injuries – to mid-70s.

In 2015, Waterloo Uncovered’s excavation of the battle site at Château d’Hougoumont, the farmhouse defended by the Coldstream Guards, discovered musket balls from some of the first shots exchanged between the French and allied troops and evidence of the close-quarters fighting.