Millions of adults who want to buy additional National Insurance (NI) to top-up their state pension entitlement have been given more time to pay at the current price – before it's due to rocket by 50% on Monday.

However, you must still contact the Government's Pension Service before then to get the reduced rate.

The extension, granted by the Department for Work and Pensions, follows huge delays experienced by many in accessing the Government’s online state pension forecasting system this week which left thousands unable to work out whether buying extra NI will boost their retirement income.

The price of buying an extra year’s NI in most cases is due to soar from £421 a year to £626 a year on Monday, which is the start of the 2009/10 tax year. However, anyone who requests a pension forecast before Monday, but receives the forecast later, can pay at 2008/09 rates if payment is made within a month of receiving the forecast.

To get the full Basic State Pension, you need to have paid National Insurance (or received qualifying benefits) for 30 years if you reach state pension age after 5 April 2010. If you reach or will reach pension age before then, men need to have paid NI for 44 years and women for 39 years.

However, many people can buy Class 3 NI contributions to make up any shortfall. Our State Pension Boosting calculator will help you work out how long you need to live to make your money back if you buy extra NI.

You can get an online State Pension forecast from the Pension Service. Yet the service will be closed between 7pm-8pm and 9pm-11pm tonight and between 11.30pm on Sunday and 12pm on Monday. Instead, its phone lines are open until 8pm tonight, and between 8am to 4pm on Saturday and Sunday, on 0845 3000 168.

The Pension Service says in a statement: “We are very sorry that some customers have been experiencing difficulties with our forecasting service. Due to unprecedented demand for our usually reliable service, some people have experienced delays. We are doing everything possible to prevent further problems occurring.”