A leaked audio recording has revealed Liz Truss said British workers needed "more graft" and lacked the "skill and application" of foreign rivals.
The Tory leadership frontrunner was chief secretary to the Treasury when the conversation with officials was recorded five years ago.
In the clip, leaked to The Guardian, Ms Truss said those outside London were less likely to be hard workers.
She said British workers as a whole also lacked the "skill and application" of foreign rivals, specifically those in China.
Ms Truss said it was "partly a mindset or attitude thing" and there seemed little desire to change the working culture so the UK could become more prosperous.
She also referenced a book she co-authored in 2012, Britannia Unchained, saying a passage from it was "mischaracterised".
The passage said British workers were among the "worst idlers in the world", but last month she claimed to have not written that chapter and blamed fellow author and current deputy prime minister Dominic Raab.
In response to the audio being leaked, a source from her team told Sky News: "These half-a-decade-old comments lack context but one thing that is as clear today as ever before is a need to boost productivity, which leads to higher wages and a better quality of life for workers right across the UK.
"As prime minister, Liz will deliver an economy that is high wage, high growth and low tax."
Ms Truss, asked about the leaked recording at a hustings in Scotland on Tuesday evening, avoided addressing her language but said "we need more productivity and more economic growth" and said she wants to "unleash those opportunities" that were voted for in 2019.
Chancellor and supporter of the foreign secretary, Nadhim Zahawi, defended Ms Truss's remarks on Wednesday.
Asked if he agreed with Ms Truss's comment that British workers need "more graft", Mr Zahawi said: "Well, what Liz Truss was talking about is can we improve productivity in the United Kingdom.
"Every percentage point of productivity improvement means the economy improves, we can pay for more nurses and doctors and teachers, as we see in this brilliant school with the holiday activities programme that they are running here, and all other services... and of course have that safety net for those people who need it most.
"That's what Liz is all about."
Ms Truss's damning comments in the leaked audio about those outside London will draw particular ire after she was forced to make a U-turn earlier this month on plans to cut civil service pay outside the capital.
She claimed at the time there had been a "wilful misrepresentation" of her policy but ended up abandoning proposals for regional pay boards for civil servants or public sector workers.
This was despite her campaign publishing specific details.
The audio emerged just moments before she appeared at a hustings in Scotland alongside her rival, Rishi Sunak. She told the audience of Scottish Tory members she was a "child of the Union" as she tried to attract their votes.
In the leaked recording, she said: "If you look at productivity, it's very, very different in London from the rest of the country.
"But basically this has been a historical fact for decades.
"Essentially, it's partly a mindset and attitude thing, I think. It's 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.
"There's a slight thing in Britain about wanting the easy answers.
"That's my reflection on the election and what's gone before it, and the referendum - we say it's all Europe that's causing these huge problems, it's all these migrants causing these problems. But actually what needs to happen is more, more graft. It's not a popular message."
Labour's shadow work and pensions secretary, Jonathan Ashworth, said Ms Truss' comments were "grossly offensive".
"Earlier this month, Liz Truss suggested workers outside London should have their pay cut," he said.
"Now it's revealed she has been saying people outside London don't have the right culture or attitude.
"With wages shrinking thanks to Tory failure to bring inflation under control and years of lacklustre growth, it's grossly offensive for Liz Truss to effectively brand British workers lazy.
"I would have hoped she had moved on from the days of her 'Britannia Unchained' fiasco, but it seems that is the blueprint for her prospective government."