The pledge by the developed world to write off $40bn of debt owed by 18 mainly African governments is likely to have a limited impact on their long-term prosperity, according to research by Standard & Poor’s, the credit rating agency.

The finance minister of the G8 countries pledged to write off the debt in June, ahead of the meeting of the countries’ political leaders that started in Gleneagles in Scotland on Thursday. The debt relief is estimated to save the countries a combined $1.5bn a year.