Virtually all small and medium-sized charities believe that the increase in Royal Mail costs for first and second class post will have a negative impact on their business.

Nine out of ten charities with less than 250 employees (88 per cent) predict the changes will have a detrimental effect, and half expect to send less post as a result.

Royal Mail price changes come into effect today, and include a substantial increase in the price of first class stamps (up from 46p to 60p) and second class stamps, and less significant increases in the price of franked mail.

Research conducted by print and mail house Pitney Bowes found that many charities are also unclear about what the changes entail, and nearly one in five do not know where to get extra information.

Earlier this year Ofcom removed price controls on bulk mailing and the HMRC exemption on charities paying VAT on their post ended earlier this month. The Direct Marketing Agency predicted that the price changes could cost charities an additional £18m a year.