The decision to stop Uber operating in London was "disproportionate" and has put thousands of jobs at risk, the prime minister has told the BBC.
The taxi-hailing company has been told it will lose its private hire licence, after Transport for London cited public safety and security concerns.
Uber is appealing against the decision but has apologised for its "mistakes".
Mrs May said Uber had questions to answer but the decision to stop it operating had "damaged lives".
Uber: Mayor backs talks after firm's apology
In one of a series of interviews ahead of the Conservative Party conference in Manchester next week, Mrs May told BBC London political editor Tim Donovan she blamed London Mayor Sadiq Khan - who is also the chairman of TfL.
"At a stroke of a pen, what the mayor has done is risked 40,000 jobs and of course... damaged the lives of those 3.5 million Uber users.
"Yes there are safety concerns and issues for Uber to address, but what I want to see is a level playing field between the private firms and our wonderful London taxis, our black cabs, our great national institution.
"I want to see a level playing field. I think a blanket ban is disproportionate."
She said: "What I think people want to see is choice."
London's transport authority, TfL, took the decision last Friday not to renew Uber's licence to operate in the capital beyond the end of September, saying the company was not fit and proper to hold a London private hire operator licence.
It cited concerns about Uber's treatment of criminal offences, medical certificates, and drivers' background checks.
But London's mayor asked TfL to meet Uber's chief executive, Dara Khosrowshahi, after he said in an open letter that Uber would appeal against the city's decision but accepted the company "must change".
Mrs May was also asked about the Grenfell Tower disaster.
She said there was a need to "get to the bottom of the truth of what happened" but also make sure that, in future, when tenants complained about safety, they were listened to.
"That's why the housing minister is going around the country, listening to social housing tenants. We want to listen to them, we want to ensure that in future, when they raise their voice, their voice is heard," she said.
Sixty six people who died in the fire have now been formally identified - the total number of estimated deaths is 80, although police have said it may be lower.
In other BBC interviews, Mrs May addressed an issue referred to by Labour leader Jeremy Corbyn in his own conference speech - the UK's housing crisis, particularly for young people.
The prime minister told the BBC's South East Today programme's Helen Catt: "We need to show how we are addressing the issues that we know are of concern to people, like housing for young people. You'll be hearing from the government, in due course, obviously, how we are going to address those issues.
"We need to ensure that young people are able to build a better future for themselves and don't fear that they are going to have a worse future than their parents."
In his closing speech at this week's Labour conference, Mr Corbyn pledged to curb rents in the private sector and said any redevelopment of a housing estate under a Labour government would have to be backed by a council ballot of residents.
Conservative Party members will gather in Manchester next week for the party's annual conference, against the backdrop of Mrs May having lost her parliamentary majority in a snap election and continued questions about cabinet unity over Brexit.
Asked about her future, Mrs May told the BBC: "I've been very clear. I've said I'm not a quitter - but there's a job for this Conservative Party in government to be doing and that's what we're getting on with.
"At this Conservative Party conference, we'll be setting out very clearly how, as Conservatives in government, we will build the road for a better future for people."