A British engineer who works on anti-hacking systems at Google has furiously accused the UK and US spying agencies of "industrial scale subversion of the judicial process" by tapping the company's internal networks.

Mike Hearn, who says he worked for two years on the networks that replicate Google data between its different computing centres, says that "GCHQ [the British surveillance centre] turns out to be even worse than the NSA [the US National Security Agency]". He added that he joined an American colleague, Brandon Downey, "in issuing a giant fuck you to the people [at the NSA and GCHQ] who made these slides".

His complaint follows the revelation by the Washington Post of slides leaked by Edward Snowden which show that GCHQ tapped the private networks between Google's centres in order to monitor traffic.

Hearn, a senior engineer at Google since 2010, complains that "nobody at GCHQ or the NSA will ever stand before a judge and answer for this industrial-scale subversion of the judicial process".

The Washington Post slides show that GCHQ has been tapping into private optic fibre cables, which Google leases from Level 3 Communications to coordinate its data stores between Finland, Dublin and Belgium, in order to monitor traffic and extract data. Hearn says that one of the slides "shows a database recording a user login as part of this [anti-hacking] system" – itself prima facie evidence that the tapping occurred.

While Google has declined to comment publicly on the revelation, Hearn's post has been widely circulated by other staff at Google, suggesting a groundswell of anger inside the company over the actions of GCHQ and the NSA.

"We designed this system to keep criminals out. There's no ambiguity here," Hearn wrote on his personal Google+ page. He added that the warrant system, with monitoring allowed through judges, "represents as good a balance as we've got between the need to restrain the state and the need to keep crime in check. Bypassing that system is illegal for a good reason."

Google is understood to be working on "forward encryption" for its private network so that communications even over its private leased lines would be unintelligible to anyone without the "keys" to decrypt it.

Google, like other companies that offer "cloud" services such as online email and storage, maintains multiple data centres in different locations so that customers can get the fastest possible access to their data: even with fibre-optic cables, the delay in accessing data from a centre on the other side of the Atlantic could create unacceptable delays.

Putting centres near to users cuts down that delay, but creates the problem of ensuring that the data held in different centres is the same. That requires large amounts of data to be passed between the centres in order to keep them synchronised. In effect, that means that information put into any one centre will be passed on to the others – meaning that tapping data flowing between any two centres can give a picture of the information flowing into any of them.