The Queen saw the funny side when a mobile phone rang at a crucial moment as she officially opened a £22 million hospice building.

The 96-year-old monarch, joined by the Princess Royal, was making a rare public appearance outside Windsor Castle, to formally launch the new home of the Thames Hospice, which has been welcoming royals for decades.

Just as she was introduced to Graham White and his wife Pat, who has stage 4 cancer and is receiving respite care at the centre in Maidenhead, his phone rang and he reached into his pocket to turn it off.

The Queen quipped: “Typical, a phone goes off immediately”, and Mr White sheepishly said it was his son ringing.

Mrs White sat as she chatted to the Queen, who stood and leaned on her now familiar walking stick, and afterwards the 63-year-old described the encounter as “very emotional”, adding: “This is a memory that I will treasure.”

She joked about her husband’s phone ringing, saying: “I could have killed him! People think the Queen is all stiff upper lip but she has a sense of humour.”

Mr White, 67, from Sandhurst, Berkshire, said: “I turned my wife’s phone off and I could have sworn mine was off – that was a bit embarrassing.”

His wife said: “The Queen said the building is beautiful and she showed a keen interest in the different treatments for cancer, and hoped the new building would help support all the cancer patients here.

The former Windsor site of Thames Hospice was opened by the Queen in 1987 and past visitors have included Diana, Princess of Wales, and Sophie, Countess of Wessex.

Now sited beside a picturesque lake surrounded by trees in nearby Maidenhead, the hospice’s new building features an open plan reception with an airy atrium, a cafe and a shop.