The defence secretary, Gavin Williamson, has urged veterans to continue to pass their stories on to future generations, as the 100-day countdown to the 75th anniversary of D-day begins.

More than 2,000 troops will take part in the commemorations, in logistical and ceremonial support, in military bands, flypasts and on Royal Navy ships escorting a specially chartered ship carrying D-day veterans to ceremonies in Portsmouth and Normandy.

Speaking onboard HMS Belfast in London, where four Royal Navy D-day veterans were awarded France’s highest honour, Williamson said the official commemorations on 6 June were “incredibly important”.

The four veterans, all from London, were awarded the Légion d’Honneur by the French ambassador Jean-Pierre Jouyet in a special ceremony.

The Royal Navy veterans Dennis Haley, John Nicholls, Charles Kavanagh and Patrick Reardon with Jean-Pierre Jouyet (left), Gavin Williamson and the director-general of the Imperial War Museum, Diane Lees, onboard HMS Belfast in London. Photograph: Kirsty O’Connor/PA

HMS Belfast was the flagship for part of the Allied armada and fired on German positions on D-day. The ship, now a branch of the Imperial War Museum, hosted the medal presentation in its ward room.

Denis Haley, 92, was at 17 a signalman on HMS Southward Ho and one of a team who towed parts of the Mulberry harbours from Portsmouth to Arromanches-les-Bains, then remained off the beach as part of a flotilla of small ships fitted with Esso smoke-making equipment until mid-July 1944.

Asked what he most remembered, he replied: “Mostly the noise, it was overwhelming, 24 hours a day. [The] noise of gunfire. The noise of the ships. You have to hear it to appreciate it.”

The fourth special service brigade makes its way on to Juno beach at Saint-Aubin-sur-Mer on the morning of 6 June 1944. Photograph: War Archive/Alamy Stock Photo/Alamy Stock Photo

Also honoured were Charles Kavanagh, 92, an able seaman who helped land tanks on Sword beach on D-day; Patrick Reardon, 93, a seaman in the RN combined operations on HMS Sheffield who landed on Omaha beach on 6 June; and John Nicholls, who served on HMS Argonaut on D-day, fired on and destroyed German gun batteries on Normandy and drove landing craft from ship to shore delivering troops and supplies.

Nicholls remembered being told of the D-day plan with just four hours to go and arriving in France to see “all hell” break loose.

“I looked at some of those troops, and as they were going in I thought: I wonder how many of them are going to come back.” He lost 65% of his hearing from the noise of explosions during the battle. “I’ve come out of it with just half of my hearing gone, but those poor devils … they lost their lives. I think of them all the time,” he

said.

Jouyet said it was a “very great honour” for him to be onboard HMS Belfast to express France’s gratitude. “At a time when Europe was dominated by a terrible dictatorship, France was able – from the first few hours of the war – to count on the support of its closest partner. The commitment to the law and democratic principles on which our societies are based inspired a shared battle for freedom. Long live Franco-British friendship,” he said.

Referring to the 75th anniversary commemorations, Williamson said: “It’s important that we remember the enormous sacrifice that was made, the enormous courage that was shown, the fact that so many people were willing to step up.”

One of the Légion d’Honneur medals given to four Royal Navy veterans who helped liberate Normandy in 1944. Photograph: Owen Cooban/Ministry of Defence/Crown Copyright/PA

He added: “As so many nations come together on the 5th and 6th of June, not just in Portsmouth but also in Normandy, the real people who are at the centre of those commemorations and celebrations about how we liberated Europe are going to be those veterans who were willing to give everything for the freedoms that we enjoy today.

“What I think is important to do is to retell the stories that they have, continue to tell the next generation [about] the contribution that they made in fighting for the values that we do hold so dear to our heart today.”