Princess Diana used to leave Kensington Palace alone at night to hand out money to sex workers and check how 'the homeless were coping', former royal press secretary Dickie Arbiter has claimed.

Speaking to the Daily Mail's Palace Confidential, Arbiter said the princess felt the night-time trips were 'something she had to do', though they were also driven by loneliness and 'boredom'.

Arbiter, 86, sat down with host Luke Blackall to mark the release of his new book, On Duty with the Royals: The Inside Story of the Modern Monarchy, which draws on his 12 years as a royal press secretary.

He worked closely with King Charles, then Prince of Wales, and Princess Diana from 1988, through their separation and divorce, and was in charge of the media operation for her funeral in 1997.

Arbiter claimed that behind the gates of Kensington Palace the princess was 'very lonely', and she would give her protection officer the slip to set out on these secret mercy missions.

He said: 'All she had was a telephone to phone friends. Her friends were reluctant to go through the security hoops [to see her].

'She had to give security her friends' names, addresses, car numbers, what time they were coming and when they were leaving. It was a bit of a rigmarole.

'She entertained people at lunch, but she didn't entertain at dinner, so she was always desperate to find things to do.

'She would try to escape, telling her protection officer: 'I'm not going anywhere tonight. You can have the night off.'

'Lo and behold, the car would come anyway and she'd drive around Paddington. Not just to hand money to sex workers, but to drive past and see how the homeless were coping.

'It was something she felt she had to do, but it was also something that she was doing out of boredom.'

Elsewhere in the interview, Arbiter criticised Earl Spencer's new memoir, Swan Song: Diana, My Sister, as 'boring', claiming it was 'long on him and short on factual accuracy'.

Spencer's book includes a string of explosive claims, most notably that Charles sounded 'giddily elated' when the pair spoke on the phone after Diana's death in a Paris car crash.

Buckingham Palace hit back with a rare statement, saying grief can 'cloud reason' and 'colour memory'.

Arbiter said he just 'does not recognise' Spencer's damning portrayal of Charles, having worked closely with him in the aftermath of Diana's death.

He said: 'I was there in the forefront. I was in charge of all media operations. There were things in [Spencer's memoir] where I thought: 'Where did he get this from?'

'I wonder why people can't just let Diana rest in peace. We have had nearly 30 years of the same things being regurgitated. There's nothing new to say.

'It's just not in Charles's DNA to be the sort of person Spencer is portraying. I saw him the day Diana returned, which was hours after she had died.

'He got off that plane and I don't think I'd ever seen him that distraught since the death of his great uncle, Lord Mountbatten.

'The man was a wreck. So for Spencer to suggest that he was like somebody who had won the lottery is just make believe. It's la la land.'

On Duty with the Royals: The Inside Story of the Modern Monarchy by Dickie Arbiter is out now. 

Subscribe to Palace Confidential on YouTube to watch the full interview.