The state second pension was introduced in 2003 as a way to help low earners and carers get more from the state pension.

Around 20 million people, the vast majority of whom are private sector workers, are currently contracted into the scheme.

TUC research found that workers on an average income of £26,000 a year and with a long work history retiring in 2030 will be £29 a week or £1,508 a year worse off.

The losses are likely to increase over time with a median earner retiring in the late 2040s set to be around £40 a week (£2,080 a year) worse off than they would be under the current state pension arrangements, the report said.

Meanwhile, a low-paid worker earning £10,000 a year can expect to be £5-10 a week better off if they retire soon after the changes take place.

However, low-paid workers retiring in the 2040s will be between £18 and £32 a week worse off.

The new single tier pension which has been set at £144 a week (in 2012/13 terms) comes into force in April 2016.

The biggest gainers are expected to be those who have spent long periods out of work or doing low paid work – a group which disproportionately comprises women and the long term self-employed.

While the TUC supports the single tier pension in principle, it believes that the initial rate of £144 a week is too low.

TUC General Secretary Frances O’Grady said: “While the government is right to move towards a simple, single state pension, setting it at just £144 a week is far too low and will mean many future pensioners will be worse off.

“The government should raise the single tier pension rate, and look to raise minimum contribution rates into workplace pensions once auto-enrolment has had time to establish itself, so that fewer people lose out under the government’s pension reforms.”