Theresa May, who will become Britain's next prime minister after her only rival in the Conservative Party leadership contest pulled out, has pledged to overhaul corporate governance rules and make big companies pay their taxes.

May, the interior minister, and junior energy minister Andrea Leadsom had been the two final candidates to lead the ruling party after two rounds of voting by its lawmakers.

The contest had been due to be decided by the party's grassroots members before Leadsom pulled out on Monday. Prior to Leadsom's withdrawal, May gave a speech in which she promised not to undo Britain's referendum decision to leave the EU and to make Britain a country that will work for everyone.

"Brexit means Brexit, and we're going to make a success of it. There will be no attempts to remain inside the EU. ///We need to reform the economy to allow more people to share in the country's prosperity. We need to put people back in control of their lives, we need to give more people opportunity, and we need to get tough on irresponsible behaviour in big business. And it doesn't matter to me whether you're Amazon, Google or Starbucks. You have a duty to put something back. You have a debt to your fellow citizens, and you have a responsibility to pay your taxes," May said.