The cost of topping up national insurance contributions will rise soon
The government's online pension forecast service has been failing to cope with "unprecedented demand" in recent days.
Users have suffered delays just days before the cost of topping up national insurance contributions is set to rise.
The Pensions Service has apologised for the glitch.
The service estimates the amount of basic and additional state pension people might get, based on their national insurance contributions.
"We are very sorry that some customers have been experiencing difficulties with our pension forecasting service over the past few days," said a spokesman from the Department for Work and Pensions.
"Due to unprecedented demand for our usually reliable service, some people have experienced delays in applying for a pension forecast. We are doing everything possible to prevent further problems from occurring."
The extra demand is likely to be the result of people checking whether they have to act before rules over extra payments to secure a full pension come into effect.
Letters have been sent to those who have a shortfall in their national insurance contributions, which could mean they lose out on a full state pension.
These letters are sent annually, but this year, the cost of topping up missed contributions will increase to £12.05 a week from £8.10 a week from 6 April.
There will also be changes to the number of "missing" contribution years that people can make good.
The right to make voluntary contributions for the period between 1996 and 2002 is being withdrawn for some people. However, after April, some will be able to top up 12 years, where currently the maximum is six.
The new 12-year rule is particularly aimed at women who left the workforce to raise families or care for other relatives.
Some BBC News website readers who have tried to use the forecast service have been left frustrated by the delays.
Dee Swan, from Crawley in West Sussex, said she wanted to check for any missing contributions.
But after waiting for more than an hour on the Pensions Service helpline, she decided to go online, but found that after all the security checks, she had only a few days to check for any missing contributions.
But then she found she could not get on to the system at all.