Theresa May and Emmanuel Macron will mark the signing of the armistice to end fighting in the first world war with a meeting at a Somme memorial to the tens of thousands of people who died on the conflict’s bloodiest battlefield.

The British prime minister and the French president will have lunch at the town hall in Albert, northern France, on 9 November. They will then lay a wreath at the memorial in Thiepval, commemorating the men from British and Commonwealth forces who died in the battle of the Somme in 1916 and have no known graves.

The event will have particular significance for Macron, who was born in nearby Amiens and grew up in an area of France scarred by the conflict. His little-known British great-grandfather fought at the Somme and was decorated for bravery.

George William Robertson, a Bristol-born butcher, stayed in France after the war and records show that in May 1919 he married the French national Suzanne Julia Amélie Leblond in Abbeville, north-west of Amiens.

Their middle daughter, Jacqueline, born in June 1922, married André Macron, a railway worker. Their son, Jean-Michel, is the French president’s father.

Macron is embarking on a five-day armistice commemoration journey to pay tribute to those who died on all sides in the conflict, which was fought between 1914 and 1918.

He will begin on 4 November in Strasbourg, where he will meet the German president, Frank-Walter Steinmeier, and finish in Compiègne on 11 November, where he will host a Franco-German ceremony with Angela Merkel, the German chancellor.

Macron has said growing up surrounded by the war cemeteries of the conflict strengthened his view that the EU is a vital block against nationalism.

Thiepval was a key strategic position controlled by the Germans during the battle of the Somme and was reduced almost entirely to rubble.

At 45 metres (148ft) high, the Thiepval memorial is the largest Commonwealth war memorial in the world. It commemorates the 72,000 unidentified soldiers from British and Commonwealth – mostly South African – forces who went missing at the Somme before 20 March 1918. The remains of about half were recovered but could not be identified. The rest have never been found.

Almost 90% of the names on the memorial were killed during the 1916 battle of the Somme, in which one million men were injured or killed. On the first day alone, the British army reported 57,500 casualties, including 19,240 deaths.

In July 2017, shortly after Macron took office, the Daily Mirror revealed it had traced his great-grandfather to the UK. Following the war, Robertson worked at his in-laws’ hotel in Amiens.

His daughters, Simone, Jacqueline and Odette, were aged under eight when Robertson abandoned his young family and moved to Paris, where he is believed to have worked for the perfume company Elizabeth Arden. He and Leblond divorced in 1928 and Robertson eventually returned to the UK, settling in Forest Gate, east London, and cutting ties with his French family. He died at home in 1956 aged 68.

Jacqueline Macron died in 1998 when the president was 21, but is not believed to have told many in her family about her British connections.

Macron, who was close to his maternal grandmother, Manette, is not thought to have publicly spoken about his British-born great-grandfather. But he told Tom Parry, a Daily Mirror journalist: “This is so wonderful. Thank you very much for doing all of this work. I never knew about this so I am delighted to be able to see these documents about this man’s life.”

In an interview with Ouest-France newspaper on Thursday, Macron said there were dangerous echoes of the interwar period emerging in Europe.

“Our future will be built at a European level … A Europe reconciled, more sovereign and more multilateral,” he said.

“Europe faces a risk: that of being dismembered by the leprosy of nationalists, and of being battered by exterior powers and thus losing its sovereignty.”