Acting Prime Minister Winston Peters says the resignation of UK Foreign Secretary Boris Johnson is "very sad" - and he expects a "spill" against UK Prime Minister Theresa May.

Mr Johnson resigned over differences with Ms May over her Brexit proposals, with Mr Johnson reported to have described them as 'like polishing a turd'.

Mr Peters said this was a loss, calling Mr Johnson a "man of true character".

"It's very sad on a personal basis because I think he's something that's lacking in world-wide politics," he told Newstalk ZB on Tuesday.

"But here we go. I think there's more to come from Boris in the future, and this is just the beginning of what is quite a momentous time in British politics."

Mr Johnson's resignation came just hours after Brexit Secretary David Davis announced his resignation and just moments before Ms May began addressing parliament about her new Brexit plans.

Mr Peters wouldn't comment if Mr Johnson would be a good prime minister, but warned he expected a leadership challenge against Ms May.

"There's every likelihood that there's a spill on," he told Newstalk ZB.

"It's been coming for a while because on the 23rd of June 2016 there was a decision, and the establishment has refused in the main to come to terms with it."

Newshub.