Donald Trump says Brexit deal will block US-UK trade
Speaking to reporters outside the White House, Mr Trump said the deal - comprising the Withdrawal Agreement and Political Declaration - could mean the UK "may not be able to trade with us".
The president appeared to be voicing concerns that the deal will not allow an increase in UK-US trade beyond the levels currently seen with EU membership.
Critics of Mrs May's deal say it could leave the UK having to stay too closely aligned to the EU, limiting the scope of deals that could be struck with non-EU countries in future. "Hopefully, she'll be able to do something about that," Mr Trump said.
The groundwork for an "ambitious" agreement with the US has been laid in five meetings of joint working groups, the spokesman said.
Heading to meet Mr Trump after his inauguration in January 2017, Mrs May was confident that a "new trading relationship" could be opened up between the UK and US.
The president caused Mrs May serious political embarrassment ahead of his visit to Britain in July when he said the PM's freshly-unveiled Chequers proposals could "kill" any UK-US trade deal.
In an interview with the Sun, he said would have done the Brexit negotiations "much differently" and claimed the PM had not listened to his advice.
He later heaped praise on "incredible" Mrs May but urged her: "Make sure we can trade together."
Mr Trump has been highly critical of the EU and its trade policies in the past and said Brexit would be a "great thing" for the UK.
His administration has slapped tariffs on steel and aluminium imports from the EU and threatened to impose further levies on car imports.
Administration officials have also reportedly maintained close connections with prominent British Eurosceptics, said to include former foreign secretary Boris Johnson and ex-Ukip leader Nigel Farage, who among the first foreign politicians to meet Mr Trump after he won the presidential election.