Former UK foreign secretary Boris Johnson. Photo: Reuters/Jeff J Mitchell

After former colleague Boris Johnson hijacked the opening of the Conservative Party’s annual conference with another attack on the government’s Brexit plan, British Prime Minister Theresa May and her allies called for unity on Sunday, AFP reported.

The former foreign minister, who is tipped as May’s successor, used a newspaper interview to characterize the prime minister’s plan for close economic ties with the European Union as “deranged” and “preposterous.” Johnson is among a group of eurosceptics in her party who are using the four-day gathering in Birmingham to push for a looser trade agreement.

Several backbench MPs also joined former UK Independence Party leader Nigel Farage, a leading Brexit campaigner, at a rally in the city on Sunday, calling on May to “chuck” her plan.

The prime minister is already under heavy pressure from EU leaders, who earlier this month rejected her trade plan and demanded a rethink before a summit in mid-October.