The search giant has launched a series of legal actions against the London-based firm in California, claiming four of its patents have been violated, and has also filed a separate case in the UK.
BT had been expecting such action after it launched its own case against Google in 2011.
That dispute has yet to be resolved.
"We have always seen litigation as a last resort, and we work hard to avoid lawsuits," said a spokeswoman for Google.
"But BT has brought several meritless patent claims against Google and our customers - and they've also been arming patent trolls."
The term "patent troll" describes firms that acquire patents so that they can later extract settlements from companies on infringement claims.
In 2012, New Jersey-based intellectual property owner Suffolk Technologies sued Google and US internet service provider AOL over two patents it had acquired from BT.
In addition, Steelhead Licensing - another firm which owns patents but does not produce products of its own - has filed a case against 14 handset makers and mobile networks, including Google's Motorola unit, claiming infringement of a wireless technology which used to be owned by BT.
A spokesman for the British firm said it did not comment on pending litigation.
Google has struck back with claims that BT has infringed a method to allow PCs to use an internet-based telephone system based on a patent originally filed by Fujitsu.
And it says BT failed to license a system used to let computer servers prioritise data - an IBM invention now owned by the search firm.
Patent consultant Florian Mueller said this was the first clear case of Google suing another company over its patents.
Previous cases involving Motorola were filed ahead of Google buying the Razr handset maker.
BT and Google are next set to meet in court in Delaware in July for a mediation hearing about the 2011 case.
Google is suing BT, claiming the British telecoms group has infringed a number of its technologies.