London: Four thousand relatives of soldiers who fought in the battle of Passchendaele, which started 100 years ago on Monday, have joined Prince Charles, the Duke and Duchess of Cambridge and Prime Minister Theresa May, at Tyne Cot cemetery in Flanders to remember one of the bloodiest chapters in the First World War.

A majority of the 11,961 servicemen buried in the Commonwealth war grave were killed during the 100-day offensive to take the village of Passchendaele, also known as the third battle of Ypres.

Tyne Cot, the largest Commonwealth war graveyard, lies on a former German machine gun position between Passchendaele, on high ground to the west, and the Belgian town of Ypres, visible from the cemetery a few miles away.

The service will include readings of wartime poems, the diaries and letters of those who fell, including a German officer, and music from the national youth choir of Scotland, the band of the HM Royal Marines Plymouth, the Welsh Guards and the central band of the Royal Air Force, among others.

Among those attending was Tim Barrett, 64, from Hardingham, in Norfolk, whose grandfather, John Ambrose Barrett, known as Jack, was killed a hundred years ago on Monday by a machine gun, on the edge of nearby Kitcheners’ Wood.

He said he felt it was important to come, as it would be the last major commemoration marking the sacrifice of his relative. Barrett was 36 when he was shot, leaving behind his wife, Evelyn, and their four children, the youngest of whom, Elsie, it is believed he never met.

“By the time you get this, I fear that the Boche will have sent something over that has got my name and address on it, and has knocked me out: and I am writing this letter, as I don’t want to leave you, Evie, without saying goodbye,” the signalman wrote to his wife six months before he was killed.

“Well dearest, let me say this, that I never knew what happiness was, and how glorious life could be, till you and I got married. And every day and every year you have given me more joy and happiness. It was glorious at the beginning, old darling, but it got better and better as time went on, as our kiddies came to us ... Kiss them for me: they cannot realise that I shall not see them again, but let them know their Daddy loved them and meant to do his very utmost for them.”

Also at the service was the former England rugby international Lewis Moody. His great uncle, Ernest George Lovejoy, died at Passchendaele on 6 November 1917.

Moody had no idea of the link to the battle, one of the most horrifying of the war, until his mother, having carried out some research, informed him as he was taking part in films to commemorate the first world war with the Rugby Football Union at the cemetery.

Lovejoy had survived the Somme but the Tyne Cot memorial wall to the missing marks that he fell at Passchendaele. “I had not a real connection with him other than he was a relative,” Moody said. “But when I found the name I was sobbing my eyes out.”

Nicola Hudson — whose great-grandfather, George William Gallirhir, a 35-year-old coal miner from Durham, was killed at the battle in 1917 — said she felt a need to honour her ancestor by attending the service. “He was hard-working down-to-earth man who adored his wife,” she said.

“He fought for a better world and I’m living in it.” Along with his last letter, sent to his wife two months before his death, Gallirhir had enclosed a poem looking ahead to them meeting again. “Some day, who knows how soon, I will return,” he wrote. “Once more we’ll wander neath the evening sky. Or by the fireside sit.”