Ofcom said it was launching the investigation after discovering mistakes that meant Vodafone had under-estimated its 4G coverage, particularly in rural areas.

As for EE, owned by Britain’s biggest telecoms company BT Group, it said the operator had over-estimated its 3G coverage, particularly in rural areas.

Ofcom said it needs communications firms to provide accurate information because mobile coverage data is used to develop coverage maps and applications which can help consumers make informed decisions on which mobile provider to choose.

The regulator added it was aiming to gather evidence by the end of December.

EE, Britain’s biggest mobile operator, said it had already updated its 3G coverage map in line with the feedback from Ofcom, and noted its 2G and 4G networks were far larger overall.

“We apologize for any confusion this may have caused, and we continue to work to give our customers the most accurate view of the coverage available to them,” a spokesperson said.

Vodafone, Britain’s third-biggest mobile operator, said its discrepancy was due to the fact that it submitted data adjusted for the signal customers actually receive, rather than the theoretical signal strength used by Ofcom.

Shares in Vodafone were 1 percent lower at 160.5 pence at 1448 GMT. BT’s were down 1.2 percent at 223.3 pence.