The information, provided by the Department of Work & Pensions (DWP) in response to a freedom of information request from Royal London, indicates just 3,524 people claimed the national insurance credit in 2016 to 2017.
This is compared with an earlier estimate when the scheme was introduced that 160,000 carers could benefit.
The provider estimates that each year of credits would add £237 per year to a carer's state pension, or more than £4,700 over the course of a typical 20-year retirement.
Assuming more than 155,000 carers a year are missing out, this creates a total loss in excess of £700m.
In 2010, the government introduced a new system of National Insurance credits to help bridge gaps in National Insurance records.
It was targeted at carers who were spending at least 20 hours caring, affecting their ability to earn enough to pay National Insurance, but who were not entitled to the carers allowance for those doing 35 hours per week of caring, and which brings automatic credits for National Insurance, Royal London explained.
The provider, alongside Carers UK, is now calling for a more proactive approach from the government to "make sure that carers take up these valuable rights."
According to Sir Steve Webb, director of policy at Royal London and former pensions minister, "these schemes are introduced with the best of intentions, but they become no more than window-dressing if virtually nobody actually takes them up."
He said: "Governments cannot simply hope that people find the information on official websites or rely on the occasional ministerial press release.
"It is time for proactive communications with those who are meant to benefit so that far more people get the help to which they are entitled."
Emily Holzhausen, director of policy and public affairs at Carers UK, added caring for more than 20 hours per week has a big impact on someone's ability to hold down a job and pay National Insurance contributions.
She said: "The carer's credit is a good scheme but it needs much more effective publicity. Caring often impacts negatively on health, wellbeing and ability to work and yet carers' contribution to the economy is worth billions a year.
"They should not lose out financially in retirement as well."
According to Paul Gibson, managing director of Granite Financial Planning, "a scheme that only 3 per cent of eligible claimants have bothered to claim must go down as a complete failure."
He said: "Whilst the amounts concerned may be modest clearly the process to claim must be looked at as it seems pointless to continue otherwise.
"Carers undoubtedly are financially disadvantaged and making claiming easier with less red tape must be the way forward."
This is not the first state pension scheme launched by the government that didn't reach its initial forecasts.
According to documents released with the last Budget, savers didn't take advantage of the state pension top up scheme – which for 18 months allowed men aged 65 or older and women aged 63 or older to increase their pot by up to £25 a week.
Only 5 per cent of people took up the programme, which was available between October 2015 and April 2017, and was based on voluntary national insurance payments.
Launched by then pension minister Baroness Ros Altmann in October 2015, the scheme offered an "opportunity for people already retired, or reaching state pension age before April 2016, to boost their later life income."
maria.espadinha@ft.com
