Less than half of the 2million people retiring between 2016 and 2020 will get a full new flat-rate state pension of at least £148.40 a week, according to official data.
Some 1.1million will get lower amounts because they 'contracted out' of the second-tier state pension during their working lives, or interrupted National Insurance contributions to bring up children or become self-employed.
That leaves just 45 per cent able to start drawing the full state pension during those five years, according to Government figures released following a freedom of information request by financial firm Hargreaves Lansdown.
The current weekly basic pension is £113.10, rising to £115.95 a week from April, but this is topped up by additional state pension entitlements - S2P and Serps - accrued during working years.
This two-tier system will start to be phased out from April 2016 and be entirely replaced for those retiring after April 2021 by a 'flat rate' pension of between £144 and £155 a week.
The change does not mean that those who fail to get the full amount will get less than they do now, and many will get more, the Government has said.
The Department for Work and Pensions says it will be 'around £150 or over' if you have made full National Insurance contributions for 35 years - and it insists no one will lose out as a result of its changes, while many will be better off.
Government figures show that one million pensioners will receive less than 86 per cent of the full new state pension when retiring during the first five years of the new system, according to Hargreaves (see tables below).
Tom McPhail, head of pensions research at the firm, said: ‘The new state pension will ultimately be a simpler and fairer system. However in the short term it will be complicated and many people are likely to get less than they may expect.
'With the new pension freedoms meaning that they will be free to spend all their private pension savings, it is imperative that they receive a proper state pension forecast. Without this, they could get a nasty shock when they do reach state pension age’
Hargreaves said the position of retirees could be slightly worse than stated because the projections provided by the DWP are based on the current Pension Credit minimum income of £148.40.
However, the state pension in 2016 is likely to be around £155 per week, based on the assumption of two years’ worth of increases at 2 per cent a year.
Number crunching: How many will fall short of full state pension when retiring between 2016 and 202? (Source: Hargreaves Lansdown based on DWP figures)
What happens if you 'contracted out'?
Hargreaves says that for those who 'contracted out' of the second-tier state pension, a deduction would be made from their state pension entitlement to reflect the fact they have been able to build up a larger private pension using the National Insurance rebate.
But McPhail added: 'The rebate by Government is pretty widely recognised to be poor value compared to state pension you were giving up.'
He explained that those in final salary schemes who contracted out should find themselves compensated for the state pension reduction, but other private pensions might not deliver quite as much due to higher charges and worse investment performance.
McPhail also warned that some people won't have realised that their work scheme contracted them out of the second-tier state pension when they joined up.
He suggests that if people are unsure if they ever contracted out, they should either ask for a pension forecast from the Government or contact previous employer schemes to check.
The Government has set up a new pension statement service to tell people what they are likely to get and ways to boost it before they retire, but this is initially available only to those retiring in the next five years before getting rolled out to everyone else. Find out more about getting a new state pension statement here.
What happens after 2020? More people will qualify for the new flat-rate full state pension as the years go by (Source: DWP)
What does the Government say?
The DWP said that no one would lose out as a result of the state pension changes and many people would be better off.
As contracting out is being abolished, payments will become increasingly similar, but differences won't be eliminated overnight, it said.
A DWP spokesman added: 'Language about people "missing out" is utterly misleading.
'Those retiring when the new state pension is in place will get at least what they would have got for the NI contributions they have paid in the current system, and many will get more.
'Our historic pension reforms will create a simplicity that hasn't existed in decades, giving people a greater sense of economic security and peace of mind in retirement.
'The new state pension will tackle inequalities of the past, with women, carers, lower earners and the self-employed set to benefit the most.'
Sam: 13th Jan 2015 14:43:00
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