The number of adults who have improved their skills with some form of training has plunged by 4 million over the past decade to the lowest level on record, as government cuts threaten to drive up inequality and damage the economy.

In a warning that a decade of decline would undermine the life chances of millions and pave the way for weaker economic growth in future, the Learning and Work Institute said the number of adults taking part in learning had dropped by about 10 percentage points since 2010, equivalent to 3.8 million fewer adults.

The research organisation found that just one-third of adults had participated in learning during the previous three years, according to a survey of 5,000 adults across the country, marking the lowest participation rate since it began conducting annual assessments in 1996.

The findings follow steep government cuts in education since the imposition of austerity by the Conservative-led coalition government a decade ago. Public spending on adult education excluding apprenticeships has been cut by almost half, while the amount spent by companies on training workers remains low compared with other advanced economies.

Boris Johnson promised a £3bn national skills fund to help adults across the country access lifelong learning and training opportunities, in a policy designed to “level up” the British economy and improve workers’ personal wellbeing.

The Learning and Work Institute said boosting the number of adults in training was vital for driving up productivity in the UK, a key measure of economic efficiency considered crucial by economists for raising workers’ pay and living standards.

Productivity growth in Britain has stalled in the past decade, rising by just 0.3% on average per year since the 2008 financial crisis, compared with an average of 2% a year in the decade before the crash.

Finding widening gaps in participation across the country, the survey showed that about 40% of adults in south-east England had access to education in the last three years, compared with just 24% in the north-east.

Adults from lower-income households are half as likely to take part in learning than those in higher-income groups, while those who left school at 16 are half as likely to enter adult learning than those who stayed in education until at least 21.

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Stephen Evans, the chief executive of the Learning and Work Institute, warned that upheaval facing the economy as Britain leaves the EU meant access to lifelong education was more important than ever before.

“If we are to succeed post-Brexit, and if we are to boost productivity and ensure everyone can achieve their potential, we must reverse this decade of decline in adult learning,” he added.

A spokeswoman for the Department for Education said the government was investing to level up skills across the country, adding: “Our investment in adult education has supported over a million people in 2018-19 to progress into work, further study, or an apprenticeship.”