(Photo: AFP)

British Prime Minister Theresa May rebuked Donald Trump and insisted Britain could strike trade agreements outside the EU as she launched a nationwide tour on Tuesday to whip up support for the contested Brexit divorce deal.

Prime Minister May headed to Wales and Northern Ireland, hours after US President Trump said it seemed like a "great deal" for the European Union that could block Britain from forging its own trade agreements with the United States.

May has two weeks to convince the public, and, crucially, a divided parliament, before the December 11 vote in the House of Commons that risks ending in a humiliating defeat and sinking the deal.

Trump suggested May had made a mistake by signing an agreement that might impede a future trade deal between London and Washington.

"Sounds like a great deal for the EU," he said at the White House, adding: "We have to take a look at seriously whether or not the UK is allowed to trade".

"As the deal stands, they may not be able to trade with the US and I don't think they want that at all. That would be a very big negative for the deal," said the president, who is close to leading Brexiteers in the UK.

"I don't think that the prime minister meant that and hopefully she'll be able to do something about that."

- May strikes back -

Starting her tour in Builth Wells in central Wales, May insisted Trump had not rained on her parade.

"We will have an independent trade policy and we will be able to negotiate trade deals with countries around the rest of the world," May.

"As regards the United States, we have already been talking to them about the sort of agreement that we could have in the future.

"It will no longer be a decision taken by Brussels. We will have control of that and we will strike trade deals that will enhance our prosperity, enhance our economy and bring jobs to the UK."

Her spokesman said there were no plans for one-to-one talks between May and Trump at the G20 summit in Buenos Aires this weekend.

The pound was down about 0.3 percent against the dollar while London's FTSE 100 share index was down 0.4 percent in early afternoon trade.

May on Sunday closed 17 months of complex talks with Brussels by sealing Brexit arrangements with the 27 other EU heads of state and government.