Jo Johnson, the Remain-backing brother of Brexit figurehead Boris Johnson, resigned as Britain's transport minister Friday, branding a proposed EU withdrawal deal a "terrible mistake" that needed a fresh referendum.

Johnson, who campaigned for Britain to stay in the European Union in the 2016 referendum, follows his elder sibling Boris - who spearheaded the pro-Brexit camp and quit as foreign secretary in July over Prime Minister Theresa May's plans - out of government over the divisive issue.

" It has become increasingly clear to me that the withdrawal agreement, which is being finalised... will be a terrible mistake," Jo Johnson wrote in a stinging resignation statement.

" It is entirely right to go back to the people and ask them to confirm their decision to leave the EU and if they choose to do that, to give them the final say on whether we leave with the prime minister's deal or without it, “ he added.

May's Downing Street office said on Twitter: "We will not under any circumstances have a second referendum".

May is reportedly close to securing a divorce deal with Brussels, as time runs out to get an agreement approved by British and European parliaments ahead of the UK's planned departure from the EU on March 29 next year.