The singers have both arranged to donate blood as part of the NHS Blood & Transplant campaign, which runs until Sunday, June 19.
Ellis-Bextor said: "Giving blood is something I've wanted to do for a while and World Blood Donor Day is the perfect opportunity.
"When I heard about the many mothers who receive blood during labour it really hit home how important it is for me to do my bit to help. I'm pleased to be able to contribute something that could save a life and plan to become a regular donor."
Frost added: "We all want to do our bit to help other people and giving blood is one of the easiest ways you can do that! I've seen first hand how blood from donors can save someone's life and am determined to make regular donations and want to encourage everyone else to do the same.
"You can make a massive difference to someone's life or even save it and it will take less than an hour of your time. Having blood in a hospital, if we were ever to need it, is something we take for granted and we shouldn't... so go on, please make a date to donate!"
Kym Marsh, Kate Thornton, Jayne Torvill, Sara Cox, Gary Lineker and Ian Wright are among other stars backing the campaign, which aims to encourage 10,000 new people to join the blood donor register.