"GCHQ practically never responds to stories about its operations. But the implications of this one are severe. There would be no way for the NSA and GCHQ, which are joined at the hip, to continue their partnership if GCHQ was willing to interfere in the US political process.
On Friday, 10 Downing Street said it had received assurances from the White House that it will not repeat the allegation, which suggests that the White House did not realize the implications of what it said.
The context matters here. Spicer repeated Napolitano’s allegation for the same reason Napolitano made it: to defend Trump’s evidence-free assertion, on 4 March, that Obama had Trump’s team placed under surveillance.
Spicer did so while reading off a long list of news reports, both credible and not, about aspects of surveillance intercepts related to Trump and Russia. Spicer’s implication is that if Trump was wrong – which he did not concede – it was because the journalists calling attention to Trump’s error lack credibility. Not a single credible news account Spicer read supports Trump’s 4 March claim. Napolitano’s did.
The context of Trump’s 4 March claim matters as well. Trump accused Obama of felonious surveillance – presidents cannot legally order their political opponents spied upon – to discredit evidence that Russia sought to aid his election.
That evidence of Russian interference, and the fallout from it, has been endorsed by three US intelligence agencies. It prompted Trump’s national security adviser to resign. It made his chief of staff, Reince Priebus, inquire about enlisting the FBI to refute news reports. It caused his attorney general, Jeff Sessions, to recuse himself from official inquiries. And the point of Trump’s claim, unequivocally denied by Obama, is to say all of this is smoke conjured by Trump’s political opponents.
One option always available to the White House is to forthrightly concede [sic] Trump was wrong."
https://www.theguardian.com/us-news/2017/mar/17/donald-trump-gchq-obama-wiretapping-claims-uk-us-alliance