By Hannah Goff BBC News education reporter

Millions of adults are economically inactive

Government skills adviser Chris Humphries says birth rate falls in the 1990s will lead to a workforce "black hole", unless urgent action is taken.

This can be filled only by targeting hard-to-reach, out-of-work people who do not receive benefits, he says.

The government said it was aware of the changing demographics of the workforce.

Economists argue that each decade a minimum of an extra 1.3 million people need to join the workforce to ensure economic growth.

The big question that is unanswered is what we are going to do to fill that demographic time bomb gap Chris Humphries

Over the past decade half of these jobs have been filled by young workers coming through the education system, and the rest by women returning to work, immigrants and there being fewer long-term sick people.

But population projections by government actuaries show there will be 600,000 fewer potential young workers aged between 16 and 24 by 2020 than there are now, and 700,000 fewer than in 2010.

This is because the birth rate fell in the mid-1990s to below what was needed to ensure population stability.

This means there are likely to be two million unfilled jobs in the British economy by 2020, says Mr Humphries, who is also the director general of qualifications group City and Guilds.

He says the government-commissioned report into skills by Sir Sandy Leitch, which called for a major increase in the skills of the existing workforce by 2020, neglected to address this issue.

'Important issue'

"The big question that is unanswered is what we are going to do to fill that demographic time bomb gap so that the gaps in the workforce are occupied in 2020 by people with the right level of skills."

He argues that the only answer is to bring a proportion of the millions of adults who are not currently in the workforce or in receipt of benefits.

"It has to be done - or the economy will not move forward. We won't get the tax receipts," he added.

The Leitch report had been right to try to call for as many young people who were not in education, employment or training to improve their skills, he said.

But this along with workforce training would not be sufficient.

"Leitch hasn't talked about that issue at all.

"This is a really important issue. Ensuring we have the workforce we need in 2020 is about ensuring that you and I live well in old age and our children have access to the education and services they deserve in a world that's becoming much tougher.

"We have got the policies coming into force around young people, those in work and those who are registered unemployed but we do not have the policy initiatives in place for those who are not employed - that will be necessary to produce the economic growth needed from 2020," he added.

Assumptions

The only answer was to campaign positively, to offer financial and tax incentives to the non-employed so that they were encouraged and incentivised to retrain and come back to work, Mr Humphries said.

A spokeswoman for the Department for Education and Skills said it was wrong to assume that changing age patterns would create a "shortfall", as the workforce the UK needed was not a fixed quantity.

"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.

"People without jobs will never achieve sustained employment without the opportunity to up-skill; Leitch recognised this in his recommendations for integrating employment and skills."