The Prime Minister said an advisory board of former defence secretaries, chiefs of staff and military specialists would bring together ideas for nationwide commemorations of the Great War.
He told an audience at the Imperial War Museum that his ambition was to have a "commemoration that captures our national spirit in every corner of the country".
"A commemoration that, like the Diamond Jubilee celebrations this year, says something about who are as a people," he continued.
More top news
The reason for the shooting is unclear, but local media have reported it may be a settling of scores between criminals.
Most parts of the country will experience near unbroken sunshine after a chilly start to the day.
As the investigation into Khalid Masood continues, former acquaintances tell of violent incidents in the murderer's past.