Luci Hindmarsh, 35, said the children's entertainer turned up late in a scruffy dress and a very badly fitting wig.
The mum, co-owner of the blog MumsMakeLists, had booked the performer for her daughter Misty's five-year-old birthday party but she and her friends were left disappointed by the look-unlike!
In a photo Tweeted by Luci, 'Elsa's' jet-black hair is clearly visible under her ill-fitting blonde wig and her silvery blue gown is hanging off her shoulders to reveal a black top underneath.
She also sang a 70s rock song as well as tunes from the hit Disney movie.
Luci, from Hackney, London, told the Mirror: "The whole point was this year we would get something magical before she gets too grown up and we could sit back and watch the magic unfold.
"But she arrived late, she came in her own clothes, and then disappeared off and when she came out she had literally put the dress on over the top of her own clothes.
"I think the photos say it all. She looked dreadful - like she'd left a rave at 6am.
"Add to this that the lady had a heavy Spanish accent and you begin to get a picture of the level of care put into providing us with an Elsa entertainer.
"One of the little girls asked 'where is Elsa?' and she said 'Elsa couldn't make it, I'm here instead'. That grated on me the most."
- Luci McQ (@LuciMcQ) February 1, 2015
Luci and husband Adam, 40, booked the impersonator after finding AEIOU Parties' website and was reassured when they sent her a photo of an impressive look-a-like.
Luci posted a photo of the unconvincing doppelganger on her personal website, which has been viewed more than 30,000 times.
One mum wrote said: "Genuinely appalled at @AeiouParties - be ashamed of yourselves! Princesses spread magic, not shatter dreams!?"
Another said: "Looks like she just rocked up from a night out! Shame on @aeiouparties"
Luci has since been refunded 50 per cent of the fee but she said: "It's not about the money. They ruined my daughter's birthday party."
A spokesman for AEIOU Parties in London said: "We have nothing really to say about the party. We've already talked about it with her. We don't really want to get involved any more in all this."
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