A handful of the largest employers, with 120,000 or more workers, must place eligible workers into pension schemes, with firms gradually being enrolled in a staging process over the next six years.
More than half a million people will be newly saving into a workplace pension by Christmas, according to Government estimates.
Savers will typically need to put aside just over £2 a week to get them started, according to Nest, a not-for-profit pension scheme set up under the new rules.
In the first four years of the scheme, workers contribute a minimum of 0.8% of earnings which works out at around £2.37 a week for someone on an average annual salary of around £20,000, Nest found.
Based on this average, employers will contribute nearly £3 per week as well and almost 60p will be added in tax relief, meaning the total going in is just under £6 a week, or around £25 a month or £309 a year.
But by 2018, as the minimum contribution increases, employees will be putting aside around £12 of their pay every week, in return for almost £9 from their employer and nearly £3 in tax relief, leading to average annual contributions of £1,235, Nest said.
Automatic enrolment aims to tackle growing concerns about an old-age poverty
crisis, as people live for longer but fail to put enough away for their
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Recent official figures show that the number of private sector workers paying into a pension is at its lowest since records began in 1953.
Last year 2.9 million private sector workers put money into schemes, the first time active membership has dipped below three million.
Joanne Segars, chief executive of the National Association of Pension Funds (NAPF), said: "The UK is drifting towards an iceberg when it comes to paying for its old age, and we need radical reform like this."
NAPF said its research shows that only a quarter (24%) of workers earning £14,000 or less save into a workplace pension.
"Crucially, this reform will reach those who have no pension: the young, the
low-paid and those working for small businesses," Ms Segars said.
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Estimates of opt-out rates are varied, although the Government believes the reforms will eventually lead to between six and nine million people newly saving or saving more in all forms of workplace pensions.
TUC general secretary Brendan Barber said: "With this Government and the last helping ensure a wide consensus around the reform package, we have some certainty that we are now at the beginning of a pensions new deal. Of course it can and should be made better, but we now have what should be a stable framework."
Some analysts have said the Government should go further in encouraging people to save, for example by making pensions more flexible so that workers can take some cash out if they need to or by increasing tax-free Isa allowances.
Ros Altmann, director-general of the firm Saga which provides insurance, holidays and financial services to people over 50, recently said that people's confidence in pensions has been knocked by scandals, disappointments for people whose pensions have not turned out as they expected and low annuity rates which have "left many pensioners receiving much poorer value for their pension savings than ever before".
The UK's pension system is "the most complex in the world" and filled with jargon, she said.
Pensions Minister Steve Webb said: "We are proud to be introducing this truly historic change which will radically alter the way we save for our old age and see millions more people putting something aside for the future.
"From October we will start seeing large firms, such as banks and big supermarkets, automatically enrolling their staff into a workplace pension. Between now and 2018 more and more employers will come on stream, right down to the smallest ones."
An advertising campaign featuring TV stars such as Theo Paphitis and Nick
Hewer was recently launched to raise awareness of the scheme which will
automatically enrol anyone aged over 22 who earns more than £8,105 and who
works for a company which is affected.
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