Britain has done the wrong thing in allowing Huawei to supply it with 5G equipment because China cannot be prevented from exploiting the technology for mass surveillance, according to a senior former Australian spy.

Simeon Gilding, a director of the Australian Signals Directorate until December, said his country’s intelligence agency was unable to design cybersecurity controls that could prevent China from gaining backdoor access to Huawei.

“We developed pages of cybersecurity mitigation measures to see if it was possible to prevent a sophisticated state actor from accessing our networks through a vendor. But we failed,” he wrote in a blog for an Australian thinktank.

He said the UK was relying on “a flawed and outdated cybersecurity model to convince themselves that they can manage the risk that Chinese intelligence services could use Huawei’s access to UK telco networks to insert bad code”.

This week Boris Johnson’s government announced that the UK would allow Huawei to supply a maximum of 35% of 5G base stations and antennas, a decision immediately criticised by Washington and many Conservative backbenchers.

Australia, like the United States, has decided to ban Huawei equipment on the grounds of security risks. But the UK’s spy agencies insist the risks can be mitigated despite China’s record of state-sponsored hacking.

Gilding highlighted China’s 2017 intelligence law, which gave the country the power to direct a company to assist it in carrying out spying if requested. Putting himself in China’s position, he asked: “What could we do with that and could anyone stop us?”

He continued: “We concluded that we could be awesome, no one would know and, if they did, we could plausibly deny our activities, safe in the knowledge that it would be too late to reverse billions of dollars’ worth of investment.”

The posting was cited by a Conservative party MP Bob Seely in a debate in the Commons on Thursday afternoon. He said it led him to conclude there were “justifiable questions” over the UK’s Huawei policy “and that we all collectively need to think very carefully about this decision”.

A second MP, Stewart McDonald, the defence spokesman for the SNP, said that what Seely had described was “the equivalent of a digital Dunkirk”.

Similar criticisms were made by Mike Pompeo, the US secretary of state, who was visiting the UK. At an event on Thursday morning, Pompeo said having Huawei technology within the network was “very difficult to mitigate”.

“When you allow the information of your citizens or the national security information of your citizens to transit a network that the Chinese Communist party has a legal mandate to obtain, it creates risk,” Pompeo said.

Unhappy backbench Conservatives are trying to see if they can force the government to harden its position against Huawei and commit to eliminating the company’s technology over the next three years.

But Vodafone and BT, the owner of the EE network, have both said the rollout of 5G would be delayed by two or three years without Huawei and the costs to consumers would be higher because Huawei’s equipment is cheaper than rivals.