Donald Trump says Theresa May's Brexit agreement "sounds like a great deal" for the EU but may hinder trade between the UK and US.
Speaking to reporters before a trip to Mississippi, the president said the deal Mrs May secured with Brussels may not be good for trade.
"I think we have to take a look at seriously whether or not the UK is allowed to trade because you know right now, if you look at the deal they may not be able to trade with us," he said.
"That wouldn't be a good thing. I don't think they meant that, I don't think the prime minister meant that and hopefully she'll be able to do something about that but right now 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."
Mrs May has spent the last 20 months negotiating with the European Union on what the future relationship with Brussels will look like after Brexit.
The prime minister needs 318 votes to get her deal through the House of Commons.
A meaningful vote is set to take place on 11 December.
Labour's Tulip Siddiq, a supporter of the Best for Britain campaign, has responded to the president's comments, saying Mrs May should "ditch this deal".
"This is a major blow for the prime minister, " says Ms Siddiq.
"This a Brexit bombshell and it has blown apart the whole government's post-Brexit trade strategy.
"Even Donald Trump, not the sharpest tool in the box knows this deal is a bad deal.
"Theresa May promised a Global Britain, but her Brexit is globally unpopular. Time to ditch this deal and let the people decide through a people's vote."
Others have also used Mr Trump's comments to criticise the prime minister's deal.
Brexiteer Michael Fabricant tweeted: "Trump is spot on. It's a great deal for the EU."
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Independent MP Charlie Elphicke simply called the president's comments "troubling".
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