Following Brexit, Three, EE, Sky and Vodafone brought in mobile roaming charges while Virgin Media O2 has committed to free EU roaming.

But according to comparison site Uswitch, Fair Usage Policies (FUPs) add an extra layer of complexity which can catch users off guard.

These put a cap on the amount of mobile data people can use while on holiday before incurring a fee, regardless of their plan at home.

As an example, O2’s FUP is limited to 25GB for all customers with the charge outside of this policy at £3.59/GB.

Virgin Media’s FUP is linked to the cost of a customer’s overall monthly plan. For every £1 of the usual monthly bill, customers can access 543MB of data so if a customer pays £50 a month with unlimited data at home, they could use a maximum of 26.71GB while roaming. Anything outside of this is charged at £4.79GB.

Meanwhile, from tomorrow, GiffGaff (which piggybacks off the O2 network) is set to slash its FUP from 20GB to 5GB with customers who go over the limit abroad charged 10p per MB.

And Asda Mobile (runs off the Vodafone network) will cut the amount of data roaming customers can use from 25GB to 5GB as of 26 September. Anything outside of this will be charged at 10p per MB.

Uswitch said FUP data caps “vary wildly across providers” with Vodafone at 25GB while Three comes in at less than half at 12GB.

EE and Sky haven’t imposed a cap, though under the government’s roaming rules, there’s a £45 a month cap on data (it doesn’t include calls or texts).

The table below shows providers’ Fair Usage Policy data allowances and charges:

Catherine Hiley, mobiles expert at Uswitch.com, said: “The lack of consistency from providers on data limits is hard to understand – and in the case of Virgin Media – hard to calculate. With such a range of restrictions, it could mean that families travelling on holiday may find some members run out of data far more quickly than others.

“While 5GB may seem like a fair amount of mobile data, GiffGaff and Asda Mobile customers will soon face hefty fees if they break their limit without putting a new plan in place.

“Consumers on unlimited plans could find downsizing tough on their travels. If they’re used to regular data-heavy activities like downloading or streaming films, they may well hit the data wall much sooner than anticipated.”

Hiley urged anyone planning to use their phones abroad to check their provider’s FUP, and consider buying add-on data if they think they’re likely to reach the cap.

She added: “While on holiday, use Wi-Fi where possible on a safe and secure connection to preserve your allowance. Alternatively, consider a plan from a SIM-only provider without a Fair Use Policy, like ID Mobile. It offers a 50GB deal for £10 a month or an unlimited plan for £16 a month.”