The former England rugby star, who is married to Anne's daughter Zara, said they had planned to mark the milestone in Scotland but now alternative arrangements are being made.

However, he refused to divulge what celebrations are being planned - in case his mother-in-law is unaware.

The Queen and the Duke of Edinburgh's only daughter turns 70 on Saturday and it is likely a birthday gathering had been planned at the royal family's Balmoral estate in Scotland, where the head of state is spending the summer with Philip.

Speaking on BBC's The One Show on Tuesday night, Tindall said: "We did have plans - it would've been up in Scotland - but obviously with Covid and Aberdeen being locked down a bit, I think everything's been scaled back a little bit.

"It's a shame. I'm sure we'll do something as a family to celebrate her 70 amazing years, she's just an incredible woman in terms of how much work she can get through in the year.

"We will be doing something, as yet I don't know whether she knows - so my lips are sealed."

Anne is famed for her hard work and no-nonsense approach to life and, like the rest of the nation, has been coping with life under lockdown.

Despite the limitations of Covid-19, Anne's milestone has been marked by a TV documentary and she has also guest-edited Country Life magazine.

Anne was followed by film-makers for more than a year to make the ITV program, which featured unseen family footage and conversations with her children, Peter Phillips and Zara, and her husband, Vice Admiral Sir Tim Laurence.

Speaking about the younger members of the monarchy, she said: "The pressure that is applied to the younger members of the family is always worse, because that's what the media is interested in and that's, you know, hard sometimes to deal with."

When she guest-edited Country Life, the princess paid tribute to her parents for instilling in her a lifelong love of nature.

The princess - christened Anne Elizabeth Alice Louise - was born at Clarence House on August 15, 1950.

She married her first husband, fellow horseman Captain Mark Phillips, in 1973.

The princess married her second husband, Sir Tim, in a low-key ceremony in 1992, after her first marriage ended in divorce after 19 years.

A keen equestrian, she was voted the BBC's Sports Personality of 1971 and went on to represent Great Britain at the 1976 Montreal Games as a member of the British three-day event team.

Australian Associated Press