Kate Hoey, MP for Vauxhall, said she was “proud” of the job the Conservative PM was doing for Britain.
Ms Hoey, who backed Leave during the EU referendum campaign, believes Mrs May is doing a good job in trying times.
She told BBC Daily Politics: “I like her style. When I watch PMQs now or when I see her particularly abroad at international events, I have to say I feel quite proud that she is the Prime Minister of the United Kingdom.
“What she is doing at the moment under very difficult circumstances I think is reasonable in terms of Brexit and how she’s handled international events.”
Host Jo Coburn was so taken aback by Ms Hoey’s comments she told her she sounded like a Conservative.
Kate Hoey, a Labour MP, heaped praise on Theresa May
What she is doing at the moment under very difficult circumstances I think is reasonable in terms of Brexit and how she’s handled international events
“I think the public are fed up of every time a politician says something the other side has to oppose it,” she said.
“We have to recognise sometimes that the country comes first.”
Ms Hoey, who broke Labour ranks to support Brexit and made campaign appearances alongside Nigel Farage, voted to trigger Article 50 last week.
She did this in spite of the fact her central London constituency voted heavily for Remain.
The EU (Notification Of Withdrawal) Bill returns to the Commons for a further three days of debate during committee stage, giving MPs the chance to amend the legislation.
Pro-Remain Conservatives are seeking assurances Parliament will get a say on the "endgame" if the Brexit negotiations collapse without a deal with the remaining 27 member states.
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Ms Hoey was speaking on Daily Politics on the BBC
The Prime Minister has promised MPs and peers will be given a vote on any agreement she reaches in the talks with Brussels.
However pro-Remain MPs fear that if the negotiations fail, Britain would simply leave the EU without any agreement at all, with potentially serious consequences for the economy.
They argue that before that happens Parliament should be given a chance to vote to send ministers back to the negotiating table to try again.
The move has infuriated pro-Leave Tories who believe it is a backdoor attempt to derail the whole Brexit process.