For at least eight decades, America’s most important ally has been the United Kingdom. Whether the enemy was Nazis, Soviets or Islamic extremists, the United States and the U.K. have long enjoyed a “special relationship” — sharing intelligence, battle plans and diplomatic strategies.

With British Prime Minister Boris Johnson’s announcement that the U.K. would allow Huawei Technologies Co. to develop parts of its 5G wireless network, that special relationship may be about to change. The House of Commons will now consider Johnson’s decision, and if it rejects China’s largest telecommunications company, then the alliance can be salvaged. But even if it doesn’t, his decision is a serious blow.

In the broadest terms, Johnson’s decision signals that America’s closest ally is not fully on board in the battle against America’s most potent adversary. In a more narrow sense, there is now a good chance that the high-level intelligence sharing that has been a hallmark of the British-American relationship will be reduced because of the threat China poses from within the British network.

Since 2017, U.S. President Donald Trump’s administration has warned the British that it would be forced to re-evaluate its intelligence relationship if the U.K. allowed Huawei into its 5G network. The problem, from the perspective of the U.S. (as well as Australia, Japan and New Zealand) is that Huawei has long-standing ties to the Chinese military, and it is obliged to assist the Chinese state when it comes to espionage.

What’s more, Huawei gear can slow down or stop wireless traffic in the event of a war or national emergency. As one prominent member of the British Parliament recently said: “Only nations able to protect their data will be sovereign.”

Allowing Huawei into the British network would have a significant impact on a partnership known as the Five Eyes, under which the U.S., U.K., Canada, Australia and New Zealand share signal intelligence against common threats. U.S. lawmakers have warned that the U.S. will have to re-examine this alliance if the U.K. lets Huawei into its network. U.S. Sen. Tom Cotton and U.S. Rep.Jim Banks have introduced legislation that would prohibit the sharing of U.S. intelligence with countries that “permit operation of Huawei fifth-generation telecommunications technology within their borders.”

This doesn’t necessarily mean the end of intelligence sharing. It just means that, when the U.S. does share, it is “going to have to be confident that the British network is not compromised,” says Tim Morrison, a senior fellow at the Hudson Institute who served on the U.S. National Security Council until last fall. “I imagine we will still share information, it may not be in real time any more, and it may not be electronically.”

The end result, in the words of Klon Kitchen, a senior fellow for technology and national security at the Heritage Foundation, is that “the U.K. has fundamentally altered this relationship.”

The issue is not just technological vulnerability, says Kitchen, who worked on cyber issues during a 15-year intelligence career. “The British government has just demonstrated that it completely misunderstands the Chinese threat.”

The House of Commons could conceivably mitigate or even correct the prime minister’s error. If it doesn’t, America’s closest ally may have to find a new best friend.

Eli Lake is a Bloomberg Opinion columnist covering national security and foreign policy.