The supervisory panel also said that "technical issues" had been identified in Huawei's engineering processes, leading to "new risks in the United Kingdom telecommunications networks".
In the letter, Ryan Ding, president of Huawei's carrier business group, stated that the company "has never and will never" use its equipment to assist espionage activities for the Chinese government.
Huawei equipment is already in use in Canada in existing networks but reports say this country may follow suit in not allowing them to participate in development of the 5G net.
It comes amid growing global concerns over Huawei, after the US, Australia and other western governments effectively barred the company from participating in the roll-out of next generation 5G telecom equipment.
Huawei remains the world's largest telecom equipment maker but spying concerns may create a significant dent in its business; without access to some of the world's richest markets, the company could eventually cede its leadership to one of Nokia or Ericsson.
According to a report by Reuters, the company has previously offered & constructed security labs in Britain & Germany with the intention of rebuilding confidence in the brand and showing that the company's equipment & consumer electronics do not contain back doors or spyware for the Chinese intelligence service.
The HCSEC, located in Oxfordshire, was launched in November 2010 to help mitigate any potential risks from using Huawei technology in the UK's critical national infrastructure.
It states that Chinese organisations are obliged to "support, cooperate with, and collaborate in, national intelligence work".
No EU-wide law has been introduced on the matter to date, and any restrictions on 5G contracts for national security reasons remain a decision for individual nations.
That's according to Huawei Poland's head Tonny Bao, who told attendees at a press conference this week that, "We are ready to establish a cybersecurity centre in Poland if authorities accept this as a trusted solution".
It also recently hit Huawei and its chief financial officer with charges of bank fraud, obstruction of justice and intellectual property theft.
As infrastructure for the next generation of ultra-high speed internet, known as 5G, is being developed, USA officials want "to move the needle" on security, a source close to the administration told Politico.
The company's CFO is in Canada, in custody, awaiting extradition to the United States on charges of fraud.