This morning Work and Pensions Secretary Amber Rudd revealed the figure had been reached.
Ms Rudd tweeted an infographic with showed a counter reaching 10 million and she wrote: “New stats out today show Automatic Enrolment is a soaraway success.
“Thanks to the Conservative Party 10 million workers can look forward to a more secure and better retirement.”
The milestone was welcomed by Hargreaves Lansdown which hailed the scheme “an astonishing success”.
Tom McPhail, head of policy at the firm, said: “Everyone who is now a member of a workplace pension is taking steps towards a more prosperous and better-funded retirement.
“The fact so few have opted out so far, is testament to the effectiveness of this nudge; most people knew they needed to do this and are just glad someone has helped them make it happen.”
“There are though still some significant challenges ahead.
“The most important issue is to build better member engagement with their retirement savings.
“This will help them work towards saving the right amount every month, to make the most of their investments and to make good choices around how and when to draw a retirement income.”
Tom Selby, senior analyst at AJ Bell, said: “Successive Governments deserve credit for shepherding auto-enrolment to this historic landmark.
“But the reforms are neither complete nor working perfectly, so to declare them a ‘soar away success’ demonstrates a worrying hubris at the very highest levels of Government.
“Auto-enrolment has undoubtedly been effective in increasing the volume of people saving something for retirement.
“This is only part of the story, however - most people are still not saving enough and there remains no concrete plan to raise contributions beyond 8%.
“Not all workers are currently benefitting from auto-enrolment either.”
He added: “The UK’s growing army of self-employed workers also remain outside of scope, with the Government merely committing to using behavioural techniques to boost savings levels in this part of the labour market.
“It seems highly unlikely this gentle nudge will have anything like the same impact as the shove of auto-enrolment.”
TPR chief executive, Lesley Titcomb, said: “We have worked with employers of all sizes to ensure automatic enrolment is a success and we are delighted that 10 million people are newly saving or saving more towards their retirement.
"The vast majority of employers continue to support automatic enrolment.
"They've played a vital role in creating a savings culture by enrolling workers so that they can now expect a pension as part and parcel of their employment.”