LONDON (AP) — Former U.K. Independence Party leader Nigel Farage says he is joining up with a pro-Brexit pressure group to oppose British Prime Minister Theresa May's plan for future ties with the European Union.

Farage was a key figure in getting Britain to hold a referendum in 2016 on EU membership, and helped lead the successful "leave" campaign.

Britain is due to leave the EU on March 29, but divorce negotiations have faltered.

May's proposal to retain close economic ties has been received coolly by the EU. And it has infuriated hardcore Brexit-backers in Britain, who say it will leave the U.K. tethered to the bloc.

Farage wrote in Saturday's Daily Telegraph newspaper that he would join a campaigning bus tour by the group Leave Means Leave to oppose May's "cowardly sell-out."