EU Commissioner Mairead McGuinness on Sunday warned the UK not to appoint Brexit hardliner as a replacement after the UK's former Brexit supremo David Frost resigned on Saturday.

"If the priority is simply to appease hardliner in Conservative Party, I think we are in a very bad place," the Commissioner told Ireland's RTE radio.

"If the people are of the view that there will be no compromise, then there will be no progress and that's untenable," she added.