A generation dip means the skills base the UK needs in 2020 will have to draw on economically inactive women, ministers told
Hundreds of thousands of stay-at-home mothers will need to be retrained for work to keep the UK workforce from shrinking.
The head of Britain's largest vocational training awards body said the Leitch review of skills necessary to meet the challenges of the UK economy in 2020 overlooked the effect of the 1990s drop in the birth rate, which will begin to transform the workforce between 2010 and 2020.
Chris Humphries, the director general of City and Guilds, said the changing demographics will mean the numbers of teenagers starting work will drop for the first time.
In 2020 there will be 600,000 fewer teenagers entering the workforce than a decade before.
Combined with the normal rate of people retiring and growth in the economy, Mr Humphries said it will mean a deficit of about 1.5 million workers, based on estimates by the Office of National Statistics. Unless more adults are given the skills to help them into work, businesses are likely to find it hard to recruit workers and expand to continue the UK's economic growth.
Mr Humphries said: "It will be the first time it's happened in British history. Normally employment growth is filled by young people. Not only will we need even more adults joining the workforce, we have to bring a completely different group of people into the workforce who weren't there before."
He said unemployment rarely drops below 1 million, so the new workers would have to be found among the 7.8 million economically inactive people, which includes carers and stay-at-home parents, the long-term sick and adult students. The biggest single group among these potential workers are the 2.3 million looking after the home and children, who are mostly women.
Mothers with no qualifications are not likely to go back to work, so it would take a huge training effort for them to replenish the workforce, Mr Humphries said.
More than four in 10 people without any exam passes are economically inactive. Among those who have GCSEs, one in eight is not employed, while one in 10 degree graduates is out of the workforce.
But there will also be a battle to persuade people to return to work. A the moment, less than a third of people who are economically inactive want to find a job.
Phil Hope, the skills minister, said the policies mapped out in Lord Leitch's review would help to slow the reduction in the number of young workers.
"The workforce will continue to change size and shape in the future and it is wrong to assume that changing age patterns necessarily create a shortfall. The workforce the UK needs is not a fixed quantity," he said.
"But Leitch does recognise that training the flow of young people entering the labour market cannot be enough to meet our future skills needs; nor do we believe that inward migration can ever be more than part of the answer."
He said the Government was committed to people working for longer as well as helping the economically inactive into work to help address the reduction in the number of people of working age.