Vodafone has been singled out by the telecoms watchdog for providing the poorest geographical coverage for 3G mobile internet in the UK after breaching the terms of its spectrum licence.

Ofcom tests published on Thursday show that despite being the only British-owned operator in the country, Vodafone is also alone in having missed the legal obligation to extend 3G mobile coverage to 90% of the population by June this year.

Ofcom plans to meet with all four networks in the coming weeks in response to growing consumer unrest about signal blackspots. Evidence gathered by the Guardian from over 1,000 readers shows many are reporting a deterioration since networks began rolling out 4G services.

Vodafone is one of the largest mobile network operators in the world, but its investment in mobile internet in its home market has been behind rivals O2, which is owned by a Spanish company, the French and German joint venture EE, and the UK's smallest operator Three, which is part of a Hong Kong conglomerate.

In 2010, in exchange for increasing the lease of 3G licences from 25 years to an indefinite duration, and releasing 4G spectrum to help speed up mobile broadband, the government increased the minimum coverage obligation for 3G from 80% to 90%.

The deadline was 30 June, but Ofcom tests carried out that month showed Vodafone missed the target by 1.4%. It was the only network to fall short. The company has promised to boost its signal, but the work will not be complete until January 2014. If Vodafone misses the new deadline, Ofcom has the power to impose a fine of up to 10% of relevant turnover.

Ofcom said: "Vodafone has put in place a plan to bring itself into compliance with the 3G coverage obligation by the end of 2013. This will involve rolling out 3G to more mobile masts than Vodafone had originally estimated as being necessary. Ofcom has reviewed this plan, and believes it to be credible, but will closely monitor Vodafone's implementation of the plan."

Vodafone will upgrade about 100 existing 2G sites to make them 3G sites in the run-up to Christmas – the sites were originally planned for upgrade in the first quarter of 2014.

"Our 3G licence includes an obligation to cover 90% of the population and Ofcom is fully aware of our plans to ensure compliance by the end of this year," a Vodafone UK spokesperson said. "Our network investment stands at more than £900m this year alone and we remain on track to deliver indoor coverage across 2G, 3G and 4G to 98% of the population by 2015, two years ahead of the regulator's deadline."

Ofcom's targets were not onerous. Under the rules networks must extend a signal to areas where 90% of the UK population live and work. This is less demanding than covering 90% of the landmass.

In order to qualify masts had to be capable of delivering a signal to just one phone, at a speed of 768 kilobits per second. On a fixed line this speed would not qualify as broadband – Ofcom considers speeds of 2 Megabits per second as the minimum for home broadband.

The regulator's tests were desk-based – using the actual location of masts and mathematical calculations to check whether they would reach theoretical test points located in population centres around the UK.

Customers suffering from poor coverage at home or at work can report mobile signal blackspots on the Guardian's user-generated map, which will be updated over the coming days.