The head of Britain’s largest vocational training awarding body said the Leitch review of skills has overlooked the effect of the 1990s drop in the birth rate, which will begin to transform the workforce between 2010 and 2020.

Chris Humphries, director general of City and Guilds, said the changing demographics will mean the numbers of young people starting work will drop for the first time. In 2020 there will be 600,000 fewer young people entering the workforce than there were a decade before.

Combined with the normal turnover of people retiring and growth in the economy, Mr Humphries said it will mean an overall deficit of about 1.5 million workers, based on estimates by the Office of National Statistics.

Unless even more adults are given the skills to help them into work, businesses are likely to find it hard to recruit new 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, em-ployment 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, historically, unemployment rarely dropped below a 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 people looking after the home and children, who are mostly female.

Mothers with no qualifications were much less likely to go back to work, Mr Humphries said, so it would take a huge training effort for them to replenish the workforce.

More than 40 per cent of people without any exam passes are economically inactive. But that falls to just one in 10 people with degrees who are out of the workforce.

But there will also be a battle to persuade people to return to work: at the moment, less than a third of people who are economically inactive want to find a job.

Ministers believe the policies mapped out in Lord Leitch’s review will help to address the reduction in the number of young workers, and they challenge the view that changing age patterns necessarily create a shortfall. “The workforce the UK needs is not a fixed quantity,” one minister said.

Although the Government at Westminster has been lukewarm about the role which inward migration can play in plugging skills gaps, the Scottish Executive continues to believe it is a key part of the answer to the problem.

Last week, Tom McCabe, the Minister for Finance and Public Service Reform, announced a new tranche of cash for the Fresh Talent initiative, which attempts to persuade students from overseas to live and work in Scotland after they graduate.

Another pound;390,000 will support a number of projects in the next academic session. The Fresh Talent scheme has already led to more than 4,300 international graduates staying on in Scotland.