More than 650 British veterans are expected at the Sword Beach ceremony on Friday, and many will sail on HMS Bulwark from Portsmouth to Normandy on Thursday.
Former troops, cadets and serving members of the armed forces will earlier parade in Portsmouth.
In France, Prince Charles is to attend a memorial service to mark the taking of the strategic Pegasus Bridge.
Queen Elizabeth II and Duke of Edinburgh will mark the D-Day anniversary during a state visit to France which begins later in Paris.
The towns may have French names, but the five Allied landing beaches are still known as Utah, Omaha, Juno, Gold and Sword.
It is 70 years since D-Day, the biggest amphibious assault in military history.
On Thursday, Prince Charles will be at Pegasus Bridge, a strategic crossing which British troops captured within minutes of landing in gliders just after midnight on June 6, 1944. A mass parachute drop will take place in Ranville, the first village to be liberated.
On the anniversary itself, the Queen will head an international service of commemoration attended by royals, presidents and prime ministers.
Hundreds of veterans are here, but their numbers are dwindling. The youngest are well into their 80s. This will be the last significant anniversary most will witness.
Their stories of heroism and sacrifice, success and disaster will soon fade from living memory.
About 160,000 Allied troops crossed the English Channel in the initial D-Day assault on June 6, 1944, paving the way for the defeat of Nazi Germany.
Heads of state from 17 nations are to attend the international ceremony at Sword Beach on Friday, the easternmost of the five landing sites.
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