has warned that watering down delivery targets for first-class post to a three-day window could still cost senders more money. The hiked the to £1.70 on Monday, more than double the 76p they cost just five years ago. The price has risen eight times over the last half-decade despite failing to meet annual delivery targets.

Customers have now been warned that delivery timeframes could be further relaxed despite the unpopular rise in cost. Watchdog Ofcom has suggested that 99.5% of first and second-class mail should be required to arrive within three and five days, respectively - but Royal Mail said such a threshold could "lead to materially higher prices for customers". The watchdog also proposed lowering Royal Mail's target of delivering first-class mail on the next working day from 93% to 90% and cutting the target for second-class post to arrive within three to five days from 99.5% to 95%.

The suggestion that receiving a higher percentage of first-class mail within three days could incur even greater costs is unlikely to go down well with UK senders - more than two-thirds of whom said they thought Monday's price rise was unfair.

A survey conducted by Citizens Advice also found that one in three people would struggle to afford a book of eight first-class stamps, now priced at £13.60.

Tom MacInnes, director of policy at the non-profit, said: " are making post less affordable and will hit low-income households and older people the hardest. People from these groups are more likely to be digitally excluded and rely on mail.

"These price increases are worsened by the fact people simply can't rely on their first or second-class mail being delivered on time - proven by Royal Mail's failure to meet any annual delivery targets for half a decade," he added.

"Ofcom must act. The regulator is considering slashing deliveries and relaxing targets but these moves must go hand-in-hand with a curb on price increases. Otherwise, Royal Mail has no incentive, as a monopoly provider, to deliver the service consumers deserve."

Ofcom's consultation on the Universal Service Obligation (USO) held by Royal Mail will close this week, with a firm decision on the changes expected in the summer.

A Royal Mail spokesperson said: "UK households on average spend less than £7 a year on stamps, and customers have the choice to post letters second class for just 87p nationwide or pay more if they want a next-day service.

"Royal Mail carefully considers prices and seeks to keep them as low as possible given the increasing cost of delivering mail.

"A complex and extensive network of trucks, planes and 85,000 posties is needed to deliver across the country for a single price.

"Ofcom has recognised that reform is urgently needed to protect the one-price-goes-anywhere universal service which requires Royal Mail to deliver letters to around 32 million UK addresses six days a week."