Finance Minister Philip Hammond said on Sunday he would resign if Boris Johnson became Britain's new prime minister because he could not support a leader happy to take the country out of the European Union without a deal.

Hammond's decision underlines the strength of feeling in parliament against a no-deal Brexit, which some lawmakers and many businesses say would be catastrophic for the economy.

A loyal Conservative who has served in a number of ministerial roles, Hammond is an unlikely rebel. He said his fears over a no deal had forced him to vote against the government for the first time in his 22-year political career last week.

Britain's new prime minister is tipped to be former London mayor Johnson, who has promised to leave the EU "do or die" by an Oct. 31 deadline. That leaves him facing a tricky Brexit challenge from the moment he takes office on Wednesday.

Johnson has said he would ramp up preparations for a no deal if he becomes prime minister to try to force the EU's negotiators to make changes to the deal that Prime Minister Theresa May sealed with Brussels and UK lawmakers voted down.

But parliamentary opposition to a no deal is growing and the EU is refusing to budge over that Withdrawal Agreement.