Boris Johnson was encouraged to persuade Prince Harry to reconsider his decision to quit as a senior working royal alongside his wife Meghan Markle in 2020.

The former prime minister revealed that he was expected to give a "pep talk" to the duke during a secret meeting which took place at a UK-Africa investment summit in London's Docklands.

Fresh off an election win, Boris was told to speak to Harry after the prince made a speech revealing he and Meghan were leaving with "great sadness" but felt they had "no other option" but to leave royal life.

But Harry was "unpersuadable" at the time, despite Boris's attempt to convince the couple they were a valuable asset to the UK. The politician remarked it was "totally hopeless."

The revelation comes in a new tell-all memoir by the prime minister entitled Unleashed Unleashed which will be serialised in the Daily Mail over the weekend.

Boris admits even he knew the unlikeliness of Harry listening to him. He writes in his memoir: "A ridiculous business... when they made me try to persuade Harry to stay. Kind of manly pep talk. Totally hopeless."

After realising the choice wouldn't be overturned, the ex-PM gave thanks for Harry's efforts with the Invictus Games and praised Meghan's philanthropy, particularly her passion for promoting education for women worldwide.

A friend of the former mayor said: "He thought they were a great asset to UK plc and it was a real shame they were leaving when they were doing such great work."

The friend added that it was a private conversation between two powerful figures. They said: "It was a man-to-man conversation, they were totally alone. But Harry wasn't for turning – he was unpersuadable by that point.

"Boris succeeded in delivering Brexit but even he couldn't stop Megxit."

After making the speech, the following day Harry went to Canada to reunite with Meghan and Archie who had already left. Since then, they have moved to Montecito, California where the couple have settled down.