Conservative leadership candidate Liz Truss said British workers needed "more graft" in leaked comments.
In the recording, published by the Guardian, Ms Truss suggested Britons lacked the "skill and application" of foreign nationals.
Labour said her comments were "offensive" and "effectively brand British workers as lazy".
The foreign secretary did not deny making the comments and said productivity needed to improve.
"I don't know what you're quoting there," Ms Truss said under questioning at a Tory leadership event on Tuesday evening.
"But the point that I've always made is what we need in this country is more productivity across the country and we need more economic growth."
The leak emerged ahead of a hustings event in Perth where Ms Truss and her leadership rival former Chancellor Rishi Sunak were grilled by Scottish Conservative members.
The party's roughly 200,000-strong membership are voting for their next leader, who will succeed Boris Johnson as prime minister in September.
The Guardian report said Ms Truss made the comments in the audio when she was chief secretary to the Treasury, a role she held between 2017 and 2019.
The report did not reveal the source of the audio recording, parts of which were redacted.
In the two-minute clip, Ms Truss said British workers' "mindset and attitude" were partly to blame for them producing less per hour than their foreign counterparts, suggesting they needed "more graft".
Ms Truss said in the recording: "Essentially it's partly a mindset and attitude thing I think. Yeah, its working culture basically. If you go to China it's quite different, I can assure you...
"There's a fundamental issue of British working culture. Essentially if we're going to be a richer country and a more prosperous country, that needs to change.
"But I don't think people are that keen to change that."
Speaking on difference in productivity in the UK, she said: "If you look at productivity, it's very, very different in London from the rest of the country."
A Truss campaign source said the comments were "half a decade old" and lacked "context", while acknowledging the UK does "need to boost productivity".
"As prime minister, Liz will deliver an economy that is high wage, high growth and low tax," the source said.
The Office for National Statistics says every country has seen slower growth in output per worker since 2009 when compared with the pre-financial crisis period.
OECD figures show that in 2019, the UK came fourth highest in the rankings of GDP per hour worked among G7 countries.
Ms Truss, who has consistently led Mr Sunak in polls, has put her tax-cutting plan for boosting economic growth at the centre of her pitch to Conservative members.
The remarks by Ms Truss echo controversial arguments made in a 2012 book she co-authored, "Britannia Unchained", in which British workers were described as among the "worst idlers in the world".
Asked about it at a leadership debate last month, Ms Truss distanced herself from the contentious assessment, claiming co-author and Sunak supporter Deputy Prime Minister Dominic Raab, had written it.
Mr Raab has subsequently said the authors of the book, which also included several other senior Conservative ministers, had agreed "collective responsibility" over its contents.
Labour's shadow work and pensions secretary Jonathan Ashworth said the "Britannia Unchained fiasco" seemed to be "the blueprint" for Ms Truss's prospective government.
"Workers across the country are working all hours to keep a roof over their heads, put food on the table and provide for their families," he said.
"Liz Truss should be helping working people to cope with this cost of living crises, as Labour this week outlined we would do, not peddling this offensive nonsense."
The BBC approached Mr Sunak's team for comment on the matter but received no response.
Source: BBC
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