LONDON - British Prime Minister Theresa May Thursday marked the start of the one-year countdown to Brexit by embarking on a whistle-stop tour of England, Scotland, Wales and Northern Ireland.

She had signed Article 50 exactly a year ago, which means Britain's membership to the European Union (EU) will end on March 28, 2019.

"Today, one year until the United Kingdom leaves the EU and begins to chart a new course in the world, I am visiting all four nations of the Union to hear from people across our country what Brexit means to them," May said ahead of her tour.

She is trying to deliver the message that she is determined to ensure there will be a bright future for every part of go-it-alone Britain.

"I am determined that as we leave the EU, and in the years ahead, we will strengthen the bonds that unite us. As we leave the EU, powers will return from Brussels to the parliaments and assemblies of the UK, closer to the people we all serve and with greater ability to deliver for their needs," she said.

She said each devolved region of Britain will see an increase in their decision-making powers.

"As the prime minister, I have an absolute responsibility to protect the integrity of the United Kingdom as a whole. That means ensuring that no new barriers are created within our common domestic market and that the UK is able to meet its international obligations in the future. No prime minister could leave these things to chance," she added.

The main opposition Labour Party has said it will back a trade deal with the EU only if the deal meets the six yardsticks it has laid down, which includes Britain retaining the benefits of the European single market and customs union.

Labour's Shadow Chancellor John McDonnell said in a radio interview Thursday that the conditions were nowhere near being met. He said Labour would not vote for the deal unless the government acted sensibly and negotiated to get a pact that meets the six tests.