Madonna has cancelled the first London show of her Madame X tour, saying she is "deeply sorry" and she "must always listen to my body and put my health first".
The star was due to perform the first of 15 London shows on Monday at the London Palladium, but has pulled out because of an injury.
The Like A Prayer singer also cancelled a show in Lisbon, Portugal, earlier this week.
In an Instagram post on Saturday, Madonna said: "I am deeply sorry that I have to cancel my concert scheduled for Monday January 27th in London.
"Under doctors guidance I have been told to rest for a few days.
"As you all know, I have injuries that have plagued me since the beginning of the tour but I must always listen to my body and put my health first."
The 61-year-old pop star said the last thing she wanted to do is "disappoint my fans or compromise the integrity of my show", adding: "So I will keep going until I cannot."
She said that her next show on Wednesday January 29 will go on as scheduled, and that "anyone who purchased a ticket will be refunded for tickets purchased that evening".
"Again I am deeply sorry to disappoint anyone and please know that it hurts me more than you can imagine to have to cancel any shows.
"Thank you again for your understanding."
The cancellation of the London show comes after she apologised to fans in Lisbon for the cancelled concert.
She had previously told fans that had been warming up for her shows "very carefully" because of her injury last week, with a video showing her limbering up while wearing knee supports.
Fans who had paid more than £300 for the show expressed their disappointment on Twitter.
One supporter said: "I’m so gutted and upset.
"You’ve cancelled London on 27th January. Why?
"We’ve train and accommodation booked."
Meanwhile, other fans wished Madonna a speedy recovery.
Ticketmaster, who sold the tickets for the events, said refunds have already been issued and apologised for the inconvenience.
Madonna’s next show is due to go ahead at the London Palladium on Wednesday January 29, followed by 13 more performances at the venue before the singer takes her tour to Paris.