Eggs were taken from Natallie Evans' diseased ovaries and fertilised by Howard Johnston before the couple split up, according to the Sun newspaper.
Ms Evans, 30, from Trowbridge in Wiltshire, says implanting the embryos is her only hope of ever having her own children.
But Mr Johnston, 25, from Gloucester, is reported to be threatening to have the embryos destroyed unless she signs a legal agreement saying he will not be liable to support any children that are born.
Financial fears
The couple donated eggs and sperm at the clinic late last year, and the embryos were created shortly afterwards, said the paper.
Ms Evans said Mr Johnston signed a consent form at the time saying his sperm and any resulting embryos could be stored for 10 years - even if he died or became mentally ill.
But there was no clause about what would happen if the couple parted after the embryos had been created.
Ms Evans said after the couple split up two months ago, she offered to sign a contract promising not to name Mr Johnston as the father, nor to claim any maintenance from him.
She said he had told her that was not necessary - but then appeared to change his mind.
Clinic letter
She told the paper she had received a letter from the Bath fertility clinic saying Mr Johnston had asked for confirmation that she could not use the embryos without his knowledge.
The clinic also told her Mr Johnston had requested the embryos be allowed to perish - but said it would take no action until it heard from Ms Evans.
She is now consulting lawyers to see what her options are.
Mr Johnston told the paper he did not necessarily want the embryos destroyed - but he was determined to get an agreement ensuring he would not have to support a child.
"I don't want to get nasty about it. But if I win the Lottery next week half of it could be taken from me.
"I think I'm well within my rights to want some sort of legislation in place."
Ms Evans had her ovaries and fallopian tubes removed after the embryos were created.
She said that if she had known Mr Johnston had had doubts, she would have had her eggs frozen for later use.