Mobile phone customers using big network providers are paying around £10 a month more for the latest iPhone compared to the market average, a survey by has found.
Analysis by Which? reveals the combined average contract price for an iPhone XS is 18 per cent more expensive with Vodafone, O2, EE and Three.
The watchdog also found the UK’s three biggest mobile operators are failing to offer their customers satisfactory service despite often costing more than smaller rivals.
Vodafone was rated the worst network in the country mobile satisfaction survey after only managing one-star ratings for customer service, value for money and technical support.
One in five Vodafone customers - 18 per cent - reported poor customer service, including complaints handling and the way their queries were dealt with, while 13 per cent said the firm's technical support was poor and 19 per cent rated it as poor value for money.
EE, the UK's largest mobile network, also continues to rank among the worst providers, with 13 per cent of its customers rating it as poor value for money, and just a quarter - 27 per cent - giving it a good or excellent rating for technical support.
Some 10 per cent of O2 customers said the company provided poor value for money.
Three achieved the highest customer score among the "big four" networks for the fourth year running, despite receiving average scores in most categories.
However, 80 per cent of Three customers rated their provider as good or excellent value for money.
Of the 13 providers included in the survey, customers mostly rated smaller "virtual networks" - those that lease spectrum from the main operators - more highly, with Which? finding that they were usually cheaper for both sim-only and certain contract deals.
From July, all mobile customers will be able to switch provider by text message and firms will be banned from charging for notice-periods running after the switch date, a practice that is currently costing those caught paying for old and new services at the same time around £10 million in total each year.
Natalie Hitchins, Which? head of home products and services, said: "The continuing reign of smaller networks over the big players goes to show exactly how important customer support and value for money are to mobile users.
"If you think you are paying too much or are not getting the level of service you expect from your provider you should shop around for a better deal - you might find you save yourself some money and probably a lot of grief too."
Vodafone said: "We are sorry that Which? members aren't as happy as they could be with our service and are working hard to understand the issue and what more we can do."
An EE spokeswoman said: "We're ranked top by OpenSignal for mobile experience in the UK, which is supported by other independent reports from Ofcom and RootMetrics.