Canada's Governor General breached royal protocol by touching the Queen because he was worried she might fall on a "slippy" carpet during an official engagement.
David Johnston was seen supporting the 91-year-old by gently touching her elbow as she ascended steps at Canada House in London on Wednesday.
He did the same thing as she left the building, which she visited with the Duke of Edinburgh in honour of the 150th anniversary of the Canadian Confederation.
Speaking afterwards he told Canadian broadcaster CBC News: "Well I'm certainly conscious of the protocol.
"I just was anxious to be sure that there was no stumbling on the steps."
He added: "It's a little bit awkward, that descent from Canada House to Trafalgar Square, and there was a 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."
The royal website says there are "no obligatory codes of behaviour when meeting the Queen or a member of the Royal Family, but many people wish to observe the traditional forms".
It is generally accepted that members of the public do not touch the Queen.