Philip Hammond is set to warn that a no-deal Brexit would harm the British economy, devour a £26.6bn Brexit war chest, and risk the break-up of the UK.
The chancellor is expected to say that Conservative candidates who are vying to be the next prime minister that they must come up with a Brexit plan “B”.
If they do not, he will hint that a second referendum could be needed to break the Parliamentary deadlock.
He will also pour cold water on tax and spending pledges by the candidates.
Mr Hammond is set to say in a major speech in the City of London on Thursday that a no-deal Brexit would soak up £26.6bn that has been set aside that could otherwise be spent by an incoming prime minister.