Millions will see prices rise by up to 92% as company announces plan to start charging by the minute, rather than by the second

Millions of Vodafone pay as you go customers will see the cost of their calls rise by as much as 90% after the company said it was "simplifying" prices.

Vodafone is switching from charging per second to charging per minute. That means, for example, that a call lasting 62 seconds that currently costs 26p will, from 1 August, cost 50p – an increase of 92%.

The move has incensed some customers, with many critical of the way the price rise was communicated. Pay as you go customers received a text message telling them to go online at vodaphone.co.uk/amends to find details of the change.

Headlined: "Pay as you go pricing just got simpler" the message said: "Pay by the minute – and always know how much credit you have left. Calls on Pay as you go are getting simpler from 1 August 2013. No more complicated pricing by the second – just clear, straightforward per minute charges."

It said that if a call is one minute and 37 seconds long, it will be charged as two minutes. "That way, you'll always know exactly how many minutes you have left."

The change affects calls to UK and international landlines and mobiles.

A spokeswoman for Vodafone – which in May announced an operating profit of £12bn – confirmed that all these calls "will be rounded up to the minute", and that the company would not be rounding down the cost of calls, for example, if a call lasted one minute and two seconds, or one minute and 10 seconds.

National, local (01, 02 and 03) and other Vodafone mobile numbers are currently charged at 25p a minute, with a minimum one-minute call charge.

One customer who contacted the Guardian, Chris Alborough, called the announcement "cynical", adding: "So, I'm too thick to work in seconds as well as minutes, and Vodaphone [sic] are therefore making life easier for me. To reward themselves for being so helpful, they are, in effect, going to charge me more for my calls."

He added: "How many PAYG customers keep a check on how many minutes they have left? All the people I know on PAYG check their balance to see how much they have left in pounds and pence, and that's what Vodaphone shows me when I check my balance. This is also a further instance of changing terms and conditions against the wishes of customers – unless, of course, I have missed the clamour for this change."

On a Vodafone online forum, one poster said: "... Please don't patronise us. This is being introduced to make more money for VF [Vodafone] pure and simple. It doesn't make anything simpler other than the fact that PAYG customers will essentially pay more for calls."

Another said: "Just noticed this piece of nonsense ... This price hike probably works out quite expensive for people who only make short calls, but may not matter over much to those who make very few calls."

The Vodafone spokeswoman said: "Many of our competitors already offer this kind of price charging."

This is not the first time Vodafone has "simplified" people's bills by rounding them up; in October 2011 it did the same thing for customers on monthly contracts. Everyone who took out a contract before February 2011 had their bill rounded up to the nearest 50p.