Brits using mobile networks EE, O2, Giffgaff, and BT Mobile are unable to make calls this evening, UK time, amid an ongoing network breakdown.

Judging from complaints online, subscribers cannot make outgoing calls, and some can't receive any, either. It appears the fault lies in the BT national network.

"There seems to be what looks like a fairly major technical issue with some UK mobile networks right now," said one source.

EE said it is aware its phone network isn't working properly, and said "engineers are working on the issue and will resolve as soon as possible."

Giffgaff, meanwhile, is reporting that its customers are unable to make any outgoing calls to landlines, mobiles outside of O2's network, and calls to 0800 numbers are currently down.

Giffgaff's status update on Monday

Given that Giffgaff piggybacks on O2's network for its service, it's no surprise then that O2's customers too are reporting issues – and O2's status page reports problems with its phone masts.

"We are aware of an issue where some O2 customers may be experiencing difficulties making calls from their mobile to EE mobiles and to landline numbers," said an O2 representative.

"BT, our fixed line partner, have confirmed they are investigating a problem within their network and we apologise for any inconvenience," it added.

According to BT's status page, the telecoms giant is "investigating an issue which is preventing some customers from making and receiving calls through their BT Mobile. Our engineers are working to fix this as soon as possible. We're sorry for any inconvenience this has caused."

BT hopes to fix its systems by 2000 GMT (1200 PT). ®