UK and US citizens should be confident their intelligence agencies operate within the law, the foreign secretary has said after talks in Washington with his US counterpart John Kerry.
William Hague was speaking amid claims about listening post GCHQ's reported links to the US monitoring programme.
UK-US intelligence sharing is based on a "framework of law", he added.
Ex-CIA worker Edward Snowden has claimed US agencies gathered and shared phone and web data with its allies.
The secret Prism programme is reported to give the US National Security Agency (NSA) and the FBI easy access to the systems of nine of the world's top internet companies, including Google, Facebook, Microsoft, Apple, Yahoo and Skype. All deny giving the US government direct access to their servers.
And earlier this week, Mr Hague told Parliament that allegations that GCHQ circumvented the law to gain information on UK citizens were "baseless".
Speaking after his talks with US Secretary of State Mr Kerry, the foreign secretary said the revelations about Prism had "not been the focus" of the talks.
"The intelligence sharing relationship between the UK and the US is unique in the world, it's the strongest in the world and it contributes massively to the national security of both countries," he added.
"I think that's something the citizens of both our countries should have confidence in, in particular that that relationship is based on a framework of law in both countries, a law that is vigorously upheld."
He said: "It's a relationship we must never endanger because it has saved many lives over recent decades in countering terrorism and in contributing to the security of all our citizens."
Mr Kerry pointed out that the system had been repeatedly approved by US Congress.
He added: "I think the Secretary and I both understand the very delicate but vital balance between privacy and the protection of people in our country."
Mr Hague previously had refused to confirm or deny claims GCHQ has had access to Prism since June 2010.