A senior U.S. delegation has visited the U.K. to lobby the country against using Huawei in the country’s 5G rollout.

The U.S. officials told the British government on Monday that allowing Huawei a role in building super-fast 5G networks across the country could jeopardize intelligence sharing between the two countries.

They also said using Chinese technology would be “nothing short of madness,” showing U.K. officials a secret document they claimed contained fresh evidence of potential security threats arising from the use of Huawei technology.

“We have strict controls over how Huawei equipment is currently deployed in the U.K. The government is undertaking

Read more: UK's Huawei 5G deal sparks dramatic US intervention