The British and Irish Lion has not played since suffering a concussion in a victory over Australia in November.
Halfpenny could feature in a potentially decisive clash with Eddie Jones' side in Cardiff after he was able to train fully with the Scarlets.
The 30-year-old will train with the Wales squad on Wednesday along with Rhys Patchell (hamstring) and may be given a run-out in the Scarlets' Pro14 encounter with Benetton Treviso this weekend after a lengthy absence.
Scarlets head coach Wayne Pivac said: "To do full contact he has got to be symptom-free and Leigh has been symptom-free, he trained with us last Thursday and Friday and yesterday [Monday] he did everything, full contact, full tackling.
"We had a mini-game scenario, what we call level three which is a game scenario so there is no hiding places. He was taking high balls, getting clattered. He did everything he needed to do.
"He goes back to Wales available, we feed back information, medical team to medical team and I have passed a little note on to Warren [Wales head coach Gatland].
"Warren will make a call on what he does from there. I think we are going to get a message on who, if any, are going to get released after Wales training on Wednesday."