British holidaymakers have been warned they could be left thousands of pounds out of pocket by EU roaming charges.
Consumer rights champion and broadcaster Martin Lewis, founder of MoneySavingExpert, said UK mobile phone operators should be forced to make their rules around roaming in the European Union clearer.
The warning comes after previous consumer protections, which required operators to text customers their pricing details when they began roaming and to cap monthly data roaming fees, lapsed at the end of June.
MoneySavingExpert also said firms no longer have to provide protections against inadvertent roaming.
A number of operators reintroduced EU roaming charges after Brexit despite promising to ditch them for good.
Lewis said he did not trust the operators to regulate themselves.
What are roaming charges and where do they apply?
Roaming describes using your phone while you are abroad. Customers are "roaming" once their phone is detected on an overseas network.
Before Brexit, UK customers travelling to the European Union were able to use their monthly allowances for calls, data and texts while they were in the EU at no extra cost.
However, since 1 January 2021, when the UK officially left the bloc, British users no longer receive free roaming in the EU by law.
In addition, certain legal protections introduced for consumers after Brexit ended on 30 June this year.
This means providers no longer have to send a text with pricing information when users begin roaming, or place a monthly cap on data roaming fees, which were previously set at £45 a month.
Firms no longer have to provide protections against inadvertent roaming, if, for example, a signal from a country across a border is stronger than the one the customer is in.
MoneySavingExpert said all networks have insisted they will keep following those rules voluntarily for now.
Which mobile networks still offer free roaming?
All of the four big UK phone operators said they would not reintroduce roaming charges after Brexit, but three of them broke that promise.
EE, Three and Vodafone have all reintroduced £2 daily EU roaming charges, according to MoneySavingExpert's report.
Smaller firms Sky Mobile and Voxi have also brought in £2 daily charges for users.
Of the four major operators, only Virgin Media 02 (formed in June 2021 when Virgin Media and O2 UK merged), still offers roaming at no extra cost.
Other providers which have no extra roaming charges include Asda Mobile, BT Mobile, Plusnet and Sainsbury's.
What advice has Martin Lewis issued?
The UK operators should be forced to make their rules about roaming clearer, said Lewis.
He also called on telecoms regulator Ofcom and the government to tighten the rules after the post-Brexit protections lapsed.
He also said the operators could not be trusted to self-regulate as some had promised to not reintroduce roaming fees post-Brexit but had now done so.
“I’ve no faith in mobile firms to self-regulate," said Lewis.
"When we left the EU, they promised not to reintroduce European roaming charges… yet most of the big networks have broken that promise."
What about 'daily' roaming charges?
MoneySavingExpert accused different providers of using different definitions of a "day" of roaming, which it said was causing confusion and risking unexpected costs.
Its report said that while some operators define a day as 24 hours from first use, others define it as anything up to 11.59pm UK time the same day, which the consumer group claims means someone who signs up at 11.58pm would only get a minute’s worth of data before needing to pay again.
It says this is not explained in arrival texts.
“We need to ban a daily roaming fee charged for use 'up to 11.59pm’ without even mentioning in which time zone,” said Lewis.
“Instead, we recommend all providers must define a roaming ‘day’ as a 24-hour period from first use, clearly explain that in the arrival text, and alert customers at least an hour before the daily charges end.”
Watch: Government tightens law on phones behind the wheel