The singer said she "struggled vocally" during her first two shows at Wembley stadium on Wednesday and Thursday where she performed both nights in front of a sold-out crowd of 100,000, and that she had "even considered miming".
"But I've never done it and I cannot in a million years do that to you. It wouldn't be the real me up there," she wrote.
Apologising to fans, the balladeer added that refunds would be available if she was unable to reschedule.
Adele has in the past spoken candidly about how difficult she finds touring and hinted on Thursday that she may retire from live performances after these Wembley concerts.
"Touring is a peculiar thing. It doesn't suit me particularly well," said a handwritten note included in Thursday's concert programme, which several fans posted on social media.
"I'm a real homebody and I get so much joy in the small things. Plus I'm dramatic and have a terrible history of touring," she wrote.
"I wanted my final shows to be in London because I don't know if I'll ever tour again and so I want my last time to be at home," she added.
But her "25" tour saw her packing areas across Europe, North America, Australia and New Zealand.
In Los Angeles, she broke a record by selling out eight consecutive shows at the Staples Center.
-AFPÂ
