The Queen and Prince Philip attended an event at Canada's High Commission in London to pay tribute to the Calgary Highlanders, Royal Hamilton Light Infantry and the Canadian Scottish Regiment.
The Queen is the Colonel-in-Chief of the Calgary Highlanders and she posed for pictures with current members of the Highlanders, which is a reserve unit.
The Royal couple were greeted by Canadian High Commissioner Gordon Campbell as a crowd of onlookers looked on behind barricades _ some waved British flags.
Britain's Daily Telegraph reported that the Queen wore a white gold and diamond brooch, fashioned in the shape of the cap badge of the Highlanders.
The newspaper reported that the brooch was a token that the regiment created especially for her.
The three units are marking the 100th anniversary of the 2nd Battle of Ypres, the first time Germany used chemical weapons in the First World War.
The Telegraph quoted Campbell as saying it was significant that the Queen took part in the event.
"You can just feel, in the room, how important it is to everyone that she is here," Campbell said.
"It is really an honour and it is the sort of commemorative activity that is important to the soldiers and the service people who are involved."