Just as the performance was due to begin, the iconic concert hall revealed on social media that it had been axed.
“We re very sorry to say that tonight s concert has been cancelled due to Tony Bennett being taken ill. Apologies to disappointed fans,” the Royal Albert Hall Twitter account wrote.
“We will contact all ticket-holders as soon as we have any more information. We would also like to wish Tony a speedy recovery.”
Bennett, an American singer whose signature tune is “I Left My Heart in San Francisco”, later explained he was suffering from influenza.
“Sorry to not be singing in London tonight,” Bennett wrote on Twitter. “I love my fans in the UK and appreciate all the well wishes to get over this flu quickly.”
US Singer songwriter Lady Gaga, whose hits “Poker Face” and “Bad Romance” have topped charts around the world, reassured fans.
“Hey everybody! Don t worry about Tony, he s OK! That s just showbiz sometimes. We ll catch u next time, we got you!” she wrote on Twitter.
It was the second of two concerts the pair were due to hold in support of WellChild, a charity that supports seriously ill children.
Ahead of Monday s performance, the two had joked together in front of press.
Charity patron Prince Harry told Bennett “I can t believe you have the energy I don t know how you do it.”
Lady Gaga, 29, laughed and replied: “He runs circles round me.”