The parents of a terminally ill toddler in the UK have lost their latest legal bid to keep their son alive.

Alfie Evans, who is 23-months-old, has been battling chronic seizures since 2016 as a result of an undiagnosed degenerative brain disease.

Alfie Evans has been battling an un-diagnosed degenerative brain disease. (Supplied) (AAP)

Doctors have said they can provide no further treatment for the little boy, but his parents, Tom Evans and Kate James, have fought against the decision.

Mr Evans and Ms James asked Court of Appeal judges to rule that the toddler should be allowed to receive treatment in Italy.

Lord Justcie Davis, Lady Justice Kin and Lord Justice Moylan on Monday ruled against the parents at the hearing in London.

They also rejected a request for the case to go to the Supreme Court.

Alfie's parents have lost their latest legal bid. (AAP) (AAP)

The parents have also lost fights in the High Court, Court of Appeal, Supreme Court and European Court of Human Rights.

The couple, however, has been granted 24 hours to ask permission directly from the court.