The president said he and Varadkar discussed the issue during their Oval Office meeting.
Mr Trump was speaking in the Oval Office after greeting Mr Varadkar.
Trump said he was "surprised at how badly" the negotiations have been handled.
"I understand why they may not find the Prime Minister's deal perfectly in line with their view of the optimum future relationship", he told Sky News.
Warwick and Leamington MP will vote against "catastrophic" no deal Brexit
In the Commons on Tuesday evening, Labour Party leader Jeremy Corbyn said Mrs May should now call a general election. She will tell MPs whether she will vote for no deal or not when she opens Wednesday's debate.
Trump, who met May last summer at Blenheim Palace, England, said he had given her his ideas on how to handle the negotiation on Britain's break from the European Union but that she had not taken his advice.
European Council president Donald Tusk has indicated that the EU may be ready to offer a lengthy extension to negotiations if the United Kingdom wants to "rethink its Brexit strategy and build consensus around it". "I hate to see it being, everything being ripped apart right now".
On the prospect of another referendum, he said: "I don't think another vote would be possible because it would be very unfair to the people that won". The UK Parliament has voted today on a series of amendments related to leaving the European Union without a deal, approving the most relevant one. It's a tough situation. "There's no reason for that to happen".
MPs have voted against holding a second Brexit referendum this evening by a substantial majority of 249 votes. Would you like to express your feelings on Brexit? I mean. she's got to do what she's got to do.
Pelosi used the luncheon to make a pointed plug for immigration after just she had just described the annual event as "a tradition where we dispense with our differences, whether they're political or whether they're competitive in any other way".