In June 2017 the European Union (EU) scrapped additional charges for roaming on mobile phones when you travel to another EU country.
Roaming is when you use your mobile phone abroad.
Can I use my mobile in the EU?
Since 2017, UK consumers have, within reason, been able to use the minutes, texts and data included on their mobile phone tariffs when travelling in the EU.
The same is true for consumers from other EU countries visiting the UK.
There are fair use limits, which mean you can use your mobile phone while travelling in another EU country, but you could not, for example, get a mobile phone contract from Greece and then use it all year round in the UK.
Before the rules changed, using a mobile phone in Europe was expensive, with cases of people returning from trips to find bills for hundreds or even thousands of pounds waiting for them.
Will roaming charges return?
After leaving the EU on 31 January 2020, the UK entered a transition period during which virtually all EU rules and regulations - including on mobile phone roaming - still apply.
The transition will end on 31 December 2020.
The government's summary of its trade deal after that did not say that the ban on additional roaming charges would continue.
What it said was that the agreement "contains measures to encourage cooperation on the promotion of fair and transparent rates for international mobile roaming".
That means that, with certain restrictions, mobile operators would be able to implement roaming charges after the end of the transition period.
The government released guidance on 13 July 2020 saying: "From 1 January 2021, the guarantee of free mobile phone roaming throughout the EU, Iceland, Liechtenstein and Norway will end."
"Check with your phone operator to find out about any roaming charges you might get from 1 January 2021."
The government has already passed legislation that would provide some safeguards for consumers:
A £45-a-month limit on the amount that customers could be charged for using mobile data abroad before having to opt into further use
Requirements for customers to be informed when they have reached 80% and 100% of their data allowance
Operators would have to take "reasonable steps" to avoid customers being charged for accidental roaming in Northern Ireland, which would happen if a phone in Northern Ireland locked onto the mobile signal coming from the Republic of Ireland.
What are mobile companies planning?
Of course, just because the operators might be allowed to reintroduce roaming charges, it does not necessarily mean that they would do so.
The problem is that without the EU rules in place, the charges would depend on agreements between UK operators and their counterparts in EU countries.
While they may have such deals in place to prevent charges increasing straight away at the start of 2021, there is no guarantee that they would be able to maintain them indefinitely.
There are three factors that mean there is a reasonable chance of UK operators being able to continue to offer inclusive roaming:
Bilateral deals - so a UK operator would make an agreement with a French operator, for example, to allow inclusive roaming for UK customers visiting France and for French customers visiting the UK
Each EU country has more than one operator, so UK operators will have a choice of companies to deal with
Some of the UK operators are parts of groups that also operate in EU countries.
The four main operators in the UK declined to comment on the specifics of the commercial deals they have done with other operators, but said they did not plan to reintroduce roaming charges.
Three said it "already offers roaming at no extra cost for its customers in over 70 destinations including the US, Australia and New Zealand. We will retain this great customer benefit regardless of Brexit negotiations."
Vodafone said it had no plans to reintroduce roaming charges.
EE said: "Our customers enjoy inclusive roaming in Europe and beyond, and we don't have any plans to change this based on the Brexit outcome. So our customers going on holiday and travelling in the EU will continue to enjoy inclusive roaming."
And O2 said: "We're committed to providing our customers with great connectivity and value when they travel outside the UK. We currently have no plans to change our roaming services across Europe."