Britain's overloaded mobile network regularly leaves one in five users unable to make calls, The Mail on Sunday can reveal.

Ofcom, the regulator, claims 96 per cent of the population has good 4G coverage, allowing them to stream videos or play games online.

But damning data involving more than 25,000 tests on phone signals found the country was riddled with blackspots that condemn millions to poor signal or no service at all.

Buffering Britain, the app behind the audit, said the findings suggested up to two in five mobile phone users could not play video clips on websites or social media, and up to one in five in some regions struggled to make a phone call. The consumer group Which? claims Britain's mobile coverage is worse than in all 27 EU member countries and every other member of the G7 group of major economies.

Data usage is growing by a fifth annually in the UK, but phone operators complain that new masts are often blocked by planning officers because of local complaints.

Buffering Britain's findings match early results from another national audit that is mapping the phone network using trackers attached to council refuse lorries and trains.

Data from Dorset shows only 63.5 per cent of the county has 'good' 4G coverage from at least one operator, even though Ofcom claims a figure of 99.9 per cent.

According to the Dorset survey, just 82.2 per cent of the county road network has even 'basic' coverage from at least one mobile network operator. This means it is hard or impossible to make mobile calls across almost a fifth of the region.

Streetwave, the Government-backed data group behind the project, has found similar gaps between Ofcom's claims and the reality for customers in 130 counties and metropolitan boroughs.

Buffering Britain's audit is based on tests conducted through its phone app, with thousands signing up since it was launched last month. It said: 'Britain has worse mobile connectivity than Kazakhstan. It's an embarrassment.'

Streetwave said: 'The results from Dorset highlight a significant gap between official claims – based on models and data supplied by the industry itself – and the connectivity consumers actually experience.'

Last month, former security minister Tom Tugendhat wrote in this newspaper that the phone signal in London was worse than in Kabul a decade ago, and phones were now 'useless in vast areas of Britain'.

The trade body Mobile UK said: 'Operators are pouring billions into network upgrades, but rollouts are stalled by sluggish planning regimes and regulatory inertia.'