Stamp prices are set to rise again next month despite repeated failures by Royal Mail to meet its delivery targets. Royal Mail said on Friday the price of a second-class stamp would go up by 4p to 91p and first-class stamps would increase by 10p to £1.80 from April 7.

It means the cost of a first-class stamp has now more than doubled - up 137% - in the past six years after eight rises, while the cost of a second-class stamp has been hiked six times. The latest rises come after Royal Mail said last month it had missed delivery targets once again in the most recent quarter.

Anne Pardoe, head of policy at Citizens Advice, said: "More than half-a-decade has gone by since the company met its delivery targets and people still face a gamble, with many uncertain if their important documents or letters like medical appointments will arrive on time.

"Things only risk getting worse when cuts to delivery days and reduced performance targets come into full effect.

"Against this backdrop, Ofcom simply cannot wave through these increases any longer.

"Higher prices must come with higher standards - increases should be tied to Royal Mail's performance on the doorstep."

Royal Mail has been called in to face a committee of MPs who have raised concerns about "chaos" in the postal service since Christmas and suggestions that some letters are being delivered in "batches".

*** Ensure our latest news headlines always appear at the top of your Google Search by making us a Preferred Source. Click here to activate or add us as Preferred Source in your Google search settings. ***

Royal Mail said the stamp rises reflected the continued increase in the cost of delivery as letter volumes fell and the number of addresses increased.

Richard Travers, managing director of letters at Royal Mail, said: "We always consider price changes very carefully, balancing affordability with the rising cost of delivering mail.

"On average, UK adults now spend just £6.50 each year on stamps and there are 70% fewer letters sent than 20 years ago.

"In the meantime, the number of addresses we deliver to has increased by four million to 32 million addresses across the UK."