Three mobile is huge –Britain's fastest growing network and you can imagine that millions of customers will feel concerned.
So let's take you through what we know about this – and it is a developing situation:
Three Mobile has 9 million customers in total
Names, addresses and emails could have been accessed
Financial details could not have been obtained
This does not appear to be a typical hack. At this stage it is suspected that the main objective was not to steal data. Rather that it was to steal phones as part of a known fraud.
Over the last four weeks, Three mobile has seen about 400 high value handsets stolen through burglaries. A further eight obtained fraudulently.
It seems that suspected culprits may have targeted the companies phone upgrade database.
Its also suspected that this was done using log-ins which the perpetrator was authorised to use - but in fact has misused.
What it all means for customers
Three mobile has told me all affected customers will be contacted as soon as possible.
Early reports suggested that 6 million customers could be affected but my information is that the company is confident that the final figure will be much smaller because this data breach affected the upgrade system not the main customer database.
I'm told Three has now put a block on that system and are investigating the records.
The biggest danger now is criminals exploit this by calling or emailing people telling them they have been affected and asking them to click on a link or divulge information.
Be careful. Beware.