Over the last three years the US government has paid at least £100 million to the UK spy agency GCHQ, "to secure access to and influence over Britain's intelligence gathering programmes", reports The Guardian.
The paper says the secret payments are set out in documents "which make clear that the Americans expect a return on the investment", and that "GCHQ has to work hard to meet their demands".
The money invested into GCHQ by the NSA includes £15.5m towards redevelopments at GCHQ's site in Bude, north Cornwall, which intercepts communications from the transatlantic fibre optic cables that carry internet traffic.
The papers are the latest batch to emerge from the cache leaked by US whistleblower Edward Snowden, the former NSA (National Security Agency)contractor, who is currently in hiding in Russia with a US warrant on his head.
"GCHQ must pull its weight and be seen to pull its weight," a GCHQ strategy briefing, seen by the paper, says.
In the documents GCHQ describes Britain's surveillance laws and regulatory regime as a "selling point" for the Americans.
The papers also reveal GCHQ is "pouring money" into efforts to gather personal information from mobile phones and apps, with the agency saying it wants to be able to "exploit any phone, anywhere, any time".
The amount of personal data available to GCHQ from internet and mobile traffic is said to have increased by 7,000 percent in the past five years - but 60 per cent of all the UK's "refined intelligence" still appears to come from the NSA, according to the documents.
A Cabinet Office spokesman told The Guardian: "In a 60-year alliance it is entirely unsurprising that there are joint projects in which resources and expertise are pooled, but the benefits flow in both directions."