The latest figures from the regulator showed 5,431 firms have confirmed they registered and completed a declaration of compliance between July 2012 and December last year.
The regulator's updated forecast of the number of employers due to become subject to automatic enrolment duties by 2018 showed around 30,000 medium sized employers reaching their 'staging dates' between April and the end of 2015.
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The forecast is aimed at assisting pension and payroll software providers, and financial and business advisers with capacity planning.
Charles Counsell, executive director of automatic enrolment, said: "The clock is ticking for thousands of medium sized employers across the UK – employing 50-250 workers – who need to meet their legal duties to provide a workplace pension.
"Payroll software and pensions providers, financial and business advisers all have a role to play in preparing for the busy times that lie ahead. "
The latest wave of advertising, in newspapers, radio and online aimed at raising awareness of the changes to workplace pensions and the actions employers need to take began this week.