Kate and Fintan Arrowsmith had invited friends on Pancake Day evening for a duck pancake dinner twist.
But when they went to open the packet in Sleaford, Lincolnshire, they noticed something horrifying.
A red substance can be seen inside the airtight package.
As there was a a razor-type slit in the pancakes they believed the liquid was blood from a cut in the factory.
Tesco denies this and claims it was actually sauce.
Fintan told Lincolnshire Live: “We believe this to be human blood. The pancakes where sealed and air tight.
“There was a razor cut in the pancakes. We guess someone on production cut their finger.
“We were ill for the whole weekend at the thought we could have consumed this.”
Kate was horrified by the sight.
She added: “We could have eaten them. We were about to put them in the microwave when we noticed the red in the packaging.”
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“We believe this to be human blood”
They returned the pancakes back to Tesco and got a £10 gift voucher and written apology.
A Tesco spokesman confirmed an investigation found the “blood” is in fact sauce.
He added: “We’ve looked into this for our customer and we’re pleased to confirm the liquid on the pancake is hoisin sauce.”
The letter read: “I am so sorry you found what appears to be blood on your pancakes. I can completely understand how concerned you must be about this.
“We take the safety of all the products we sell very seriously, although I can see that your experience doesn’t reflect this.
“We know it is time consuming to complain, but we are grateful that you have, as it gives us the chance to address any issues with our products.
“They’ll take your feedback on board, look into this and do everything they can to make sure that something like this doesn’t happen again.”