First-class stamps will rise above £1 for the first time unless Saturday letter deliveries are scrapped

The price of a first-class stamp will rise “significantly” if the postal service resumes six-day delivery, Royal Mail has warned.

Chairman Keith Williams said fewer letters are being sent, meaning weekend delivery costs more.

‘The costs for us are partially driven through [the fact that the] Letter volumes have gone down,” Williams told The Telegraph.

“You deliver the same number of letters in six days when you could do it in five. So that drives up stamp prices.’

Chairman Keith Williams said a drop in the number of letters sent meant weekend delivery costs would rise (file image).

The postal service is required by law to deliver mail six days a week, but regulator Ofcom has accepted evidence that 97 per cent would like to reduce the service to five days

The cost of a first class stamp rose by 10p to 95p in March, while a second class stamp rose to 68p.

It comes after Grant Shapps told a select committee earlier this week that it would not allow Royal Mail to reduce the number of days it delivers.

The postal service is required by law to deliver mail six days a week, but regulator Ofcom has accepted evidence that 97 per cent would like to reduce the service to five days.

Postal workers have left in protest at pay and working conditions. Grant Shapps told a House of Commons committee he would not allow Royal Mail to reduce the number of days it delivers

It has struggled to modernize to compete with other delivery services like Evri and Amazon.

Mr Williams said Royal Mail would be allowed to make a profit margin of between 5 per cent and 10 per cent after nationalisation, under the rules.

“We have achieved this margin in two years since privatization,” added Mr. Williams.

The chairman said the postal service was “stuck” between the government and the regulator as it failed to meet financial sustainability targets.

The Royal Mail boss also said Mr Shapps could potentially be forced to allow the changes.

The postal service has also been hit by strikes throughout December, which is usually the busiest time.

Strikes have pushed up delivery costs for businesses across the country, and some have restricted their delivery options as a result.

Mr Williams said Business Secretary Grant Shapps could be forced to accept the changes to weekly delivery days

John Lewis charges have increased from £3.95 for standard delivery on items over £50 to £4.50 since November. Argos has increased its fast delivery fee by £1.

Retailer Oliver Bonas has suspended its click-and-collect option, giving holiday shoppers fewer options to order gifts.

A spokesman for John Lewis said the retailer is working to keep its delivery costs as low as possible.

An Argos spokesman said that their delivery costs are regularly monitored and that the company strives to provide customers with added value.