The lookalike was only shown for seconds as a BBC camera panned across the throng, but the quick glimpse was enough for eagle-eyed viewers watching at home to spark up an internet storm.
On Twitter, one viewer wrote: "Lovely to see the Queen enjoying a bit of Robert Plant at #Glastonbury."
Another quipped: "The Queen has turned up at Glastonbury to watch Robert Plant. That disguise is fooling nobody."
Other social media users joked about how, despite bobbing her head to the music, the woman didn't actually seem to be enjoying herself that much. Twitter user @andrewora5 commented, "Lizz [sic] is needing a break."
The amusing moment comes just days after the 96-year-old monarch defied her doctor's orders and took to horseback for a trot around Windsor. She was advised to stop riding nine months ago and this marks the first time she's been back in the saddle since.
A source told The Sun she had deeply missed riding, adding: "'The Queen has enjoyed being on her horse again. Riding again is a wonderful sign after those worries about her health. To do so at 96 is pretty remarkable."
Queen Elizabeth, who celebrated 70 years on the British throne recently, was unable to attend all of the Platinum Jubilee events due to unspecified "health issues" and could be seen using a walking stick for support. She has even begun using a golf buggy to attend certain royal engagements.
However, during a meeting with Archbishop of Canterbury Justin Welby at Windsor Castle on Tuesday, she was seen without the aid of a stick, fuelling further rumours her health may be improving.
