Pensions minister Steve Webb has admitted over 500,000 women would be excluded from NEST under plans to raise the earnings threshold for inclusion from £5,035 to £7,500.
His comments came after questioning from shadow pensions minister Rachel Reeves.
Webb is expected to announce the threshold will be lifted to £7,500 at a press conference at NEST at 9.30am this morning.
Plans to introduce a three-month grace period before employees are enrolled, with an option for them to join if they wish to, are also anticipated, as part of the DWP's report on the NEST review.
However, Webb's figures show that raising the threshold to £7,000 would penalise 500,000 women, or up to one million women if set at £8,000.
A threshold of £11,000 would exclude up to two million women, or just under 15% of all working women in the UK.
Any raise in the threshold would exclude up to half a million people within black and minority ethnic communities.
There has been pressure from UK plc to raise the threshold in order to reduce costs for employers.
Earlier in October, British Chambers of Commerce (BCC) director general David Frost called for the government to raise the earnings threshold to £7,500 for a three-month waiting period in order to prevent employers paying into pensions for temporary workers.
In a written reply to Reeves (pictured), Webb says: "The Pensions Act 2008 provides that from 2012 workers between the age of 22 and state pension age, with annual earnings in at least one job of more than £5,035 (2006 earnings terms) will be eligible for automatic enrolment into a qualifying pension scheme, unless they are already participating in such a scheme.
"Our analysis indicates around ten to 11 million people will be eligible for automatic enrolment in a workplace pension scheme from 2012.
"That is why automatic enrolment is so important, but it is crucial that we get the detail right, so we instigated the Making Automatic Enrolment Review to ensure that automatic enrolment is implemented in the most effective way."
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