Canada's Governor General says he was trying to prevent the Queen from stumbling when he breached royal protocol by touching the monarch on Wednesday while en route to Canada House in central London.

"I'm certainly conscious of the protocol," David Johnston told CBC News.

"I was just anxious to be sure there was no stumbling on the steps. It's a little bit awkward, that descent from Canada House to Trafalgar Square, and there was carpet that was a little slippy, and so I thought perhaps it was appropriate to breach protocol just to be sure that there was no stumble."

'I thought perhaps it was appropriate to breach protocol,' said Canadian Gov. Gen. David Johnston on Wednesday about touching the Queen's arm. Johnston said he wanted to ensure there was 'no stumbling on the steps' while they heading to Canada House in London. (Jared Thomas/CBC)

Johnston, who will be replaced as the Queen's representative in Canada by former astronaut Julie Payette in September, is in London this week for what is likely his final meeting with Queen Elizabeth before his term of office ends.

The Queen visited Canada House to mark the country's 150th birthday. She was given a sapphire brooch from the Canadian people to mark her Sapphire Jubilee, marking 65 years of her reign.

While the royal website says there are no "obligatory codes of behaviour when meeting the Queen," there are observed "traditional forms" of greeting. Members of the Royal Family have been known to give handshakes, but other forms of touching have been rare.