The firm achieved one-star ratings for customer service, value for money and technical support.

EE, the UK’s largest mobile network, also ranked among the worst providers, according to its customers. 02, Virgin Mobile and BT Mobile also scored poorly.

A Vodafone UK spokesperson sadi: “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.”

Of the 13 networks included in the survey, it was mostly smaller ‘virtual networks’ that customers rated more highly.

Giffgaff topped the poll, while Utility Warehouse and Plusnet Mobile came in second and third place respectively.

Almost all (97%) Giffgaff customers told Which? they would recommend the network to friends and family.

When it comes to value for money, the big four firms – EE, O2, Vodafone and Three – were the most expensive, according to the study.

Sim only deals were on average 31% (over £3 a month) more expensive with one of the big four providers than with one of the smaller virtual networks.

The same could be said for the combined average monthly contract price for an iPhone XS, which was 18% more (£10 dearer) with EE, O2, Vodafone and Three.

Shop around for a better deal

Natalie Hitchins, head of home products and services at Which?, 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.”

Regulator Ofcom has announced plans to allow all mobile customers to be able to switch provider by text message, hopefully making it quicker and easier for customers to change their mobile network.

From July, new rules from the regulator will mean that mobile providers will also be banned from charging for notice-periods running after the switch date.

This will stop people from paying for old and new services at the same time – something that is currently costing UK mobile customers around £10 million in total each year, according to Ofcom.