A pressing alert has been sounded for millions of holidaymakers to do a swift check to dodge a surprise bill. Jet2, Ryanair, Easyjet, TUI passengers are urged to scrutinise their phone contracts before jetting off to sidestep a hefty charge. If your mobile contract doesn't cover data roaming, you could be stung with up to £6 per GB of data used , warns price comparison site Uswitch. However, setting a roaming spending cap before you start using your data abroad can help you steer clear of unexpected bills. Expert Ernest Doku advises: "Many networks will apply additional charges if you go over your allowance or if you don't have roaming included, so setting up a spending cap ahead of your trip is a great way to avoid these." Simrat Sharma, a mobiles guru at Uswitch, suggests that switching to an eSIM - short for embedded SIM - can be cheaper than using international roaming. "eSIMs make it easier to change networks," she explained. "So for example, if you're abroad you can quickly connect to the local network to pay local rates - without having to add or swap a physical local SIM card for your device.", reports Birmingham Live . "This means travel eSIMs are almost always cheaper than using international roaming, as users are effectively tapping into the same network plans as locals. "The software can easily be accessed via your device's app store and uploaded to your phone in a few quick steps," she added. Mr Doku advised: "If in doubt, contact your provider to talk about roaming caps and what your current deal will allow you to use, as well as how much this will cost." Several of the UK's leading mobile phone providers offer customers the option to adjust plans and contracts prior to jetting off overseas. For the latest breaking news and stories from across the globe from the Daily Star, sign up for our newsletters .