Huawei is helping develop 5G networks for all four of the UK’s major mobile phone operators – even though the government has yet to confirm whether the controversial Chinese technology company will be permitted to build the next generation of wireless infrastructure.

The revelation threatens to exacerbate tensions between the UK and the US, which has taken a firm line against the company amid claims, strongly denied, that it is controlled by the Chinese government and that its equipment could be used to spy on other countries and companies.

The Observer understands that Huawei is already involved in building 5G networks in six of the seven cities in the UK where Vodafone has gone live. It is also helping build hundreds of 5G sites for EE, and has won 5G contracts to build networks for Three and O2 when they go live.

The decision to use Huawei in the “non-core” parts of their networks – chiefly the radio systems allowing wireless communication – is a gamble for UK telecom operators. They may be left counting the cost if the government bans the Chinese company from any involvement with 5G.

The consultancy Assembly suggests a partial to full restriction on Huawei could result in an 18-to-24-month delay to the widespread availability of 5G in the UK. The UK would then fail to become a world leader in 5G – a key government target – costing the economy between £4.5bn and £6.8bn.

But the US has placed the UK in a difficult position. In May, President Donald Trump ordered the US treasury department to name Huawei as a national security threat, a move that led US firms to distance themselves from the company. Three months earlier, the heads of major intelligence agencies, including the FBI, the CIA and the NSA, had warned US citizens not to use Huawei phones.

The US has also pressured other countries to stop using Huawei equipment in their national infrastructure and has warned close allies such as the UK that to continue the relationship might jeopardise its ability to share classified security information with them.

Former defence secretary Gavin Williamson. Photograph: Will Oliver/EPA

US concerns are shared by several senior UK government figures. Gavin Williamson was sacked as defence secretary this year after he was suspected of leaking confidential National Security Council discussions that suggested Huawei would be allowed to provide non-core 5G equipment to UK operators. A government review of the UK telecoms supply chain, which would signal whether Huawei should be allowed to build 5G networks, was due to be published in the spring but has yet to materialise as officials and ministers clash over the extent to which the Chinese company should be restricted.

Whitehall officials are concerned that excluding Huawei, one of the very few companies that can provide next-generation wireless technology, would have damaging implications for the future of the UK’s infrastructure. They have taken note of what happened last December when the O2 4G network went down for 24 hours due to problems with technology provided by the Swedish telecoms firm Ericsson.

“If we had banned Huawei and everyone was just using Ericsson, we would have had a day without any mobile coverage on any network – not a good position to be in,” said Matthew Howett at Assembly.

The quiet rollout of 5G telecoms infrastructure supplied by Huawei has been tracked by an enthusiast, Peter Clarke, who has earned a cult following posting pictures of the new masts on his Twitter feed.

Globally, the company has signed contracts to help build 50 5G networks, totalling around 150,000 base stations.

In recent weeks Trump has softened his position, agreeing that US firms should be able to sell some components to Huawei, a climbdown described by the Republican senator Marco Rubio as a “catastrophic mistake”.

One key issue to be resolved is what constitutes core and non-core equipment. In contrast to previous mobile phone technologies, 5G will have more sensitive information accessed closer to the edge – or the non-core – of the network, which Huawei’s critics could flag as a concern.

“There is the whole debate about where the core and access network are delineated,” said Howett. “But the reality is that the operators are all using Huawei to an extent – they are quite happy with it. The government has huge ambitions for what 5G can deliver to the economy, and a bad decision based on politics could seriously stop that from being a reality.”