One of GCHQ's top officials has accused Huawei of "shoddy" engineering, which could see it banned from Westminister and other sensitive areas in the UK.
Dr Ian Levy, technical director at the National Cyber Security Centre, told the BBC that its shortcomings could also see the Chinese phone manufacturer banned from the "brains" of 5G networks.
In February, GCHQ said the security concerns around its technologies were 'manageable', but last month said it hadn't seen anything to give it confidence that Huawei would improve its security systems.
Dr Levy said: "The security in Huawei is like nothing else - it's engineering like it's back in the year 2000 - it's very, very shoddy.
"We've seen nothing to give us any confidence that the transformation programme is going to do what they say it's going to do."
Huawei said it would address GCHQ's concerns and stressed that it has already set aside a £1.5 billion fund to do so.
Huawei is considering suing the US government after it banned federal agencies using the manufacturer over the security concerns. The US had led calls for Western countries to do the same, with Japan, Australia and New Zealand already having introduced bans.
The primary concerns lie in its security vulnerabilities, which the US claims could be hijacked by the Chinese government as a spying tool.
Mike Conaway, a member of the US House Intelligence Committee, said: "You would never know when the Chinese government decide to force Huawei... to do things that would be in the best interests of the Communist party, to eavesdrop on the US."
Mr Conaway added: "Obviously, the terrific relationship between the UK and the United States - English-speaking countries - is important to maintain.
"But as a part of that we will have to assess what kind of risks we would have in sharing... secrets that would go across Huawei equipment, Huawei networks.
"We can always share things old-school ways by, you know, paper back and forth. But, in terms of being able to electronically communicate, across Huawei gear, Huawei networks, would be risky at best."