Alex Younger, Chief of MI6, gives a speech at University of St Andrews in Glenrothes, Scotland.

The U.K.'s spy chief said that decisions still had to be made on China's role in building Britain's 5G network.

In a rare public speech, MI6 chief Alex Younger — also known as "C" — was asked Monday about allowing Chinese telecoms giants such as Huawei to provide technology for the U.K.'s 5G rollout. He told students at St Andrews University in Scotland that the Secret Intelligence Service (SIS) had "some decisions to take" over China's involvement.

"We need to decide the extent to which we are going to be comfortable with Chinese ownership of these technologies," he said, noting that Chinese frameworks varied from the U.K.'s both legally and ethically.

Superfast 5G mobile internet is widely predicted to revolutionise cities and future technologies such as autonomous vehicles, with many countries preparing for rollouts within the next few years.

Last week, New Zealand banned Huawei from providing tech for its 5G rollout — the third member of the Five Eyes security alliance to do so. At the time, New Zealand's government said it had identified a "significant network security risk."

Fellow members Australia and the U.S. have also excluded Chinese telecoms firms from providing 5G equipment for their domestic networks, leaving Canada and the U.K. as the only members not to rule out using the telecoms giant.

All three nations cited national security fears as the reason for excluding Chinese companies from their 5G rollouts, with Younger's Australian counterpart referring to them as "high-risk vendors."

A Huawei spokesperson was not immediately available for comment when contacted by CNBC. However, Eric Xu, one of the rotating chairmen at Huawei, told CNBC on Thursday that blocking Huawei from the 5G market could result in higher costs for both consumers and telecoms firms.

Huawei and ZTE – another Chinese firm blocked from the U.S. 5G market – have repeatedly denied that their involvement in the rollouts would give China's government access to international networks.