One in four homes and businesses in Burnley do not have full 4G coverage, according to Ofcom.
Figures from the communications regulator's Connected Nations report show that in January 2018, 25% of premises in the area did not have reliable 4G coverage from all four mobile phone networks. Last year, that figure was 35%.
Campaigners have said that it is essential for consumers to get access to 4G from all four networks, so they do not have to suffer localised monopolies. The County Land and Business Association is calling on the Government to introduce legally binding targets to force the mobile phone networks to extend 4G coverage, saying that mobile phone operators were letting rural areas become "a digital wilderness".
CLA senior economic adviser, Dr Charles Trotman, said: "State of the art coverage will improve productivity of businesses and make them far more profitable," explaining that the CLA's aim is to push for comprehensive and reliable 4G coverage from all four major mobile networks - EE, 3, Virgin and O2.
"Our objective is universal coverage," he said. "But everything is predicated on the operators themselves to put the infrastructure in place. We need to incentivise all operators to branch out, to give consumers more market choice and avoid localised monopolies."
Ofcom has proposed introducing obligations that require networks to deliver better quality indoor coverage. An Ofcom spokesman said: "We agree mobile coverage must improve, particularly in rural communities, and we're working with the Government and the industry to support this.
"We want mobile companies to extend their networks as a priority, and we've announced plans to make them increase coverage for rural areas as we release more airwaves next year."
Ofcom's report also measures coverage by geographical area, for someone using their phone outside. The figures show that 86% of the total area of Burnley is covered by all four 4G networks.
A spokesman for the Department for Digital, Culture, Media and Sport said: "We have implemented major changes to planning laws and made it cheaper and easier for the industry to roll out masts. But the mobile companies now need to act fast on these reforms and deliver better coverage across the UK, particularly in rural areas."
A spokesman from Mobile UK, which represents the four mobile networks, said: "Huge improvements have been made to mobile coverage in recent years, as a result of the £2 billion invested each year by the industry. For this to continue, we need the active co-operation of landowners to provide operators with suitable, cost-effective sites."
The Government target is 95% geographic coverage across the UK from at least one network by 2022.