Two couples with a long family history of breast cancer plan to be the first to screen embryos to prevent their future children developing the disease.
Embryos can be tested for disease-linked genes
Doctors would select embryos free from a gene which carries a greater risk of the cancer, but which does not necessarily mean it will develop.
Paul Serhal of University College Hospital, London, is applying to test for the BRCA1 gene.
The Human Fertilisation and Embryology Authority has agreed it in principle.
Approval may come within the next few months.
Until recently, the screening - called pre-implantation genetic diagnosis (PGD) - was approved only for genes that always cause diseases when inherited, such as in cystic fibrosis.
In 2004, the HFEA awarded experts at University College Hospital a licence to screen for a bowel cancer that does not normally develop until the late teens or early adulthood.
Last May, the watchdog ruled it acceptable for doctors to screen embryos for genes such as BRCA1, which raise the risk of cancer in adulthood by 60-80%.
However, it still has to approve each application on a "case by case" basis - something these couples are still waiting for.
Critics say if the HFEA does give the go ahead it is a continued journey down a slippery slope to "designer babies". There is concern that in the future embryos could be screened for non-lethal and even trivial conditions.
Also, the procedure means that affected embryos are created and then destroyed.
Any applications to the HFEA would go through a "peer review", being examined by other scientists before a licence would be awarded, he added.
Helen and Matthew are one of the two couples hoping to receive the test.
Helen, 22 and from Bedford, told The Times newspaper: "I have lived much of my life with cancer and death, and the fear that I might have to face it and might pass on the risk to my children.
"This gives us a chance to make sure that our daughters won't have the same experience."
Paul Serhal told Radio 4's Today programme: "What we are doing here is giving parents an option to spare their children the trauma that they had to go through and eradicate this genetic transmission that has been happening from generation to generation."
Josephine Quintavalle of Comment on Reproductive Ethics said: "This is not an acceptable way forward.It is not a solution to breast cancer and not a cure.
"It's already leading in the direction of a quest for perfection."
Dr Sarah Rawlings of Breakthrough Breast Cancer said: "The decision to use embryonic screening for diseases like breast cancer is a complex and very personal issue.
"Women with a family history of breast cancer tell us that what might be right for one woman may not be the best option for another.
"It's important for anyone affected to have access to appropriate information and support so they can make the right choice for them."
PGD involves genetically testing an embryo in a laboratory. In order to achieve this, couples have in vitro fertilisation (IVF) treatment followed by an additional genetic testing stage.