Transport minister Jo Johnson has quit the government, calling for the public to have a fresh say on Brexit.
The brother of former foreign secretary Boris said: "It has become increasingly clear to me that the withdrawal agreement, which is being finalised in Brussels and Whitehall even as I write, will be a bad mistake".
Calling for a new referendum, Mr Johnson said: "My view is that this is so different from what was billed that it would be an absolute travesty if we do not go back to the people and ask them if they actually do want to exit the European Union on this extraordinarily hopeless basis". While Johnson's resignation is likely to spur critics in the Conservative Party, securing a second referendum would also require support from the Labour Party, which is yet to fully clarify its stance on a referendum and what that could entail in terms of the question put to the public.
The shadow foreign secretary, Emily Thornberry, has attempted to calm Labour dismay at Jeremy Corbyn's announcement that "we can't stop Brexit", by insisting the option of campaigning for a second referendum was still on the table.
However he emphasised that a "no deal" Brexit, which the government insists is the only other option, would mean "chaos" that would cause disruption, delay and deep damage to the United Kingdom economy.
Mr Johnson said his role as transport minister had caused him concern about the impact of a no-deal outcome.
Boris Johnson backed his brother's decision to quit, saying: "We may not have agreed about Brexit but we are united in dismay at the intellectually and politically indefensible of the UK position".
The muddled stance came as Theresa May struggles to thrash out a deal with the European Union - amid rising fears that negotiations could fail.
Gabart attendu dimanche après-midi à Pointe-à-Pitre — Route du Rhum
La course devrait se jouer dans un chrono similaire au temps record établi lors de la dernière édition (7jours, 15 heures). Il faudra ensuite contourner l'île pour rejoindre la ligne d'arrivée, dans des conditions instables et orageuses.
Johnson's resignation - the 14th so far this year - prompted fears that others could follow suit.
"The prime minister has told colleagues this week we should aim to conclude the withdrawal agreement as soon as possible but we will not do that at any cost".
"The prime minister thanks Jo Johnson for his work in government".
Earlier, the chief of Britain's defence staff General Sir Nicholas Carter said the armed forces were making "sensible" contingency plans on Brexit and other issues.
A spokesperson from the People's Vote campaign told i they were confident that, despite Corbyn's words, the party could still be relied on to support its campaign.
Corbyn, a long-standing Eurosceptic, has seemingly been mindful that a significant amount of Labour voters opted to leave in the June 2016 referendum.
Asked by German magazine Der Spiegel if he would stop Brexit if he could, the Labour leader said: "We can't stop it".
May is trying to hammer out the final details of the British divorce deal but the talks have become stuck over how the two sides can prevent a hard border from being required in Ireland.