The Queen joined three Canadian military regiments in London, to commemorate the regiments' role in a historic First World War battle.

The Calgary Highlanders, Royal Hamilton Light Infantry and the Canadian Scottish Regiments are in Europe to mark the 100-year anniversary of the Second Battle of Ypres, where Germany used chemical weapons on a large scale for the first time.

The commemoration took place at London's Green Park on Sunday, after which Queen Elizabeth joined the regiments for a reception at Canada House, the Canadian high commission in London.

The Queen, who is colonel-in-chief of the Calgary Highlanders, was joined by her husband Prince Philip, colonel-in-chief of the Royal Hamilton Light Infantry, and Princess Alexandra, colonel-in-chief of the Canadian Scottish Regiment.

"(It's) very unusual for the Queen to attend an event in London on a Sunday afternoon, but it’s a mark of high esteem for this regiment that she's colonel-in-chief of, and also for the significance of the anniversary," said CTV News royal commentator Richard Berthelsen.

The regiments' visit to London is part of a European commemorative pilgrimage continuing the traditions of the original Battalions of the 1st Canadian Division who took part in The Battle of Saint Julien, a battle within the Second Battle of Ypres.

CTV London Bureau Chief Ben O'Hara-Byrne attended the ceremony at Green Park Sunday.

The Queen and Prince Philip arrive at Canada House to honour 3 Cdn regiments on centenary of 2nd battle of Ypres pic.twitter.com/ka8gE6XFJg — Ben O'Hara-Byrne (@BenCTV) April 19, 2015