Britain's two major parties are preparing for messy leadership contests, setting the stage for parallel competitions that will shape the nature of Britain's exit from the European Union.

Britain's decision to exit from the European Union during Thursday's referendum has thrown both the ruling Conservative and opposition Labour parties into turmoil, with Prime Minister David Cameron pledging to step down and Labour Party chief Jeremy Corbyn expected to face a challenge for the leadership.

Nominations for the Conservative Party close at noon (1100GMT) on Thursday. Ex-London Mayor Boris Johnson, Home Secretary Theresa May and Justice Secretary Michael Gove are among those who've formally announced their intention to replace Cameron.

Corbyn is expected to face a challenge after losing a lopsided no-confidence vote on Tuesday.