The number of people who believe the Prince of Wales has made a positive contribution to the Royal Family has fallen in the run-up to the 20th anniversary of Princess Diana's death.

A YouGov poll commissioned by the Press Association found that 36 per cent of the British public think Charles has been beneficial to the monarchy, compared to 60 per cent in 2013.

More than a quarter - 27 per cent - think the longest-serving heir to the throne has had a negative impact on the royals, in contrast to just 15 per cent four years ago.

The research also showed that only 14 per cent of the British public want the Duchess of Cornwall to become Queen when Charles is King.

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Most - 39 per cent - think she should take the title of Princess Consort instead, while almost a third of Brits - 30 per cent - believe she should not have any title at all.

Royal commentator Penny Junor suggested the public has still not forgiven the Prince for the breakdown of his marriage to Diana, and his standing had fallen because of the publicity surrounding the anniversary.

The Press Association-commissioned poll follows surveys by national newspapers which also found unfavourable public opinions towards Charles and Camilla.

Almost a third of the British public feel the Royal Family has shown Diana's memory too little respect.

But more than 40 per cent believe the Windsors have "got the balance about right".

YouGov research shows that 30 per cent thought the royals showed too little respect to Diana's memory, 44 per cent felt the balance was about right, 4 per cent felt there was too much respect and 22 per cent were unsure.

More than two-fifths of British adults (44 per cent) also believe the Royal Family changed for the better because of Diana.

Only 5 per cent felt the Windsors had changed for the worse, while nearly a third (32 per cent) said the royals had not changed at all.

The monarchy faced one of its most turbulent times in the aftermath of Diana's tragic death in a Paris underpass on August 31 1997.

In terms of the Princess's legacy, the research found, 20 years on, that Diana is most well known in Britain for being the "People's Princess", rather than for her personal troubles, her charity work and being William and Harry's mother.

But most younger people know her best for dying in a car crash.

The Prince of Wales and the Duchess of Cornwall last month. Photo / AP file
The Prince of Wales and the Duchess of Cornwall last month. Photo / AP file

- PA, AAP