Yahoo’s court battle to fend off NSA demands for customer data was killed off when the US Government threatened the tech giant with overwhelming non-compliance fees of £250,000 per day.

The proposed daily charges were revealed yesterday, when more than a thousand pages of documents from the 2008 Foreign Intelligence Surveillance (FISA) court case were released for the first time.

Yahoo first defended itself from the NSA’s data demands on the grounds that they were unconstitutional, being far too broad in scope – but it lost that fight in both FISA’s court and in an appeal to the Foreign Intelligence Court of Review (FISC).

The Government was able to use Yahoo’s defeat as ground to persuade other huge tech giants to give up their data to the spying programme.

Ron Bell, general counsel for Yahoo, wrote in a blog post yesterday: "We consider this an important win for transparency, and hope that these records help promote informed discussion about the relationship between privacy, due process, and intelligence gathering.

"The released documents underscore how we had to fight every step of the way to challenge the U.S. Government’s surveillance efforts. At one point, the U.S. Government threatened the imposition of $250,000 in fines per day if we refused to comply."

Most major US tech companies were named by the NSA as data providers in documents leaked by whistleblower Edward Snowden, including Google, Facebook and Apple.

Yahoo said it was still fighting for the release of more documents from the secret courts.

"Now that the FISC-R matter is resolved, we will work hard to make the materials from the FISC case public, as well," wrote Bell.