Senior figures behind efforts to curtail the powers of American spy agencies have seized on the decision by the world’s largest tech companies to call for radical surveillance reform, saying the unexpected intervention is a potential “game-changer”.

In an open letter published jointly on Monday, eight tech giants, including Apple, Google and Facebook, said disclosures by the National Security Agency whistleblower Edward Snowden revealed that basic rights and freedoms were being undermined.

The companies – which also include Microsoft, Yahoo, AOL, LinkedIn and Twitter, and have a combined value of $1.4tn – called for widespread changes that, if enacted, would end many of the current programs through which governments spy on citizens at home and abroad.

"This is a major game-changer,” Leslie Harris, president of the Center for Democracy and Technology, an advocacy group, told the Guardian.

She said the letter was certain to get the attention of the White House and Congress, not least because the often-cautious tech companies wrote the letter in unison, accompanied by personal statements from the likes of Mark Zuckerberg, the founder of Facebook, and his counterpart at Google, Larry Page.

Tech giants usually leave public lobbying to the dozen or so industry associations in Washington. It is unprecedented for the major tech giants to put their names to a single political statement of this kind.

“These are companies that have been deputised to be in the middle of a new, massive, systematic collection regime, against their will,” Harris said. “They are being told they can’t tell their users about it; they are losing business around the world and discovering that their data centres are being broken into. And I think they’re ready to fight back.”

Harris said senior executives in tech companies had coalesced around a set of political demands closely aligned with those called for by the internet freedom community, including her organisation. The Center for Democracy and Technology receives some funding from all eight tech giants behind the letter, but is a non-profit advocacy group.

The letter comes at a critical juncture. In six days, President Barack Obama will receive the findings of an independent review panel, consisting of five top-level experts appointed by the White House to propose a raft of possible reforms.

The make-up of the group, which includes Richard Clarke, a former White House counter-terrorism chief, and Michael Morell, the previous deputy director of the CIA, has been criticised as being too close to the intelligence community, but the administration insists its work has been independent and robust and will inform Obama's thinking about surveillance reform.

Next month, Congress is expected to debate competing legislation that will either substantially change how the NSA and partner agencies do business – for example, by rendering illegal the current practice of collecting phone records in bulk – or make only a few marginal changes.

The US intelligence community and its supporters on Capitol Hill are advocating only superficial changes that would leave their powers untouched and even codify practices that some legal experts argue are unconstitutional. This sets up a major legislative battle in the new year, which observers say could go either way. The position the White House takes in the debate could prove crucial.

A key barometer of change will be whether the phone records collection program survives. In their letter, the eight internet companies pointedly said governments should “limit surveillance to specific known users for lawful purposes” and “should not undertake bulk data collection”. The letter also does not distinguish between citizens and non-citizens, revealing, perhaps, that executives at the companies are acutely aware of the potential for NSA revelations to damage their business abroad.

Justin Amash, the House Republican whose amendment to end the mass collection of American citizens’ phone records was only narrowly defeated in July, said the tech companies' statement would galvanise support for the USA Freedom Act, the bill which is proposing to end bulk collection. “Businesses increasingly recognise that our government's out-of-control surveillance hurts their bottom line and costs American jobs,” Amash said in a statement.

The legislation, which is authored by Republican Jim Sensenbrenner in the House and Democrat Patrick Leahy in the Senate, and has 120 bipartisan co-sponsors in both chambers, has become the rallying point for advocates of reform on Capitol Hill. Sensenbrenner said on Monday that tech companies were among those “directly impacted by the NSA’s overreach”. The open letter, he added, “helps add to the growing momentum for much-needed reform”.

Leahy, in a statement emailed to the Guardian, pressed the case for reform in the wake of the letter. “The global competitiveness of the American technology industry has been undermined by revelations of massive dragnet surveillance programs," Leahy said. "We need to make substantial reforms to our surveillance laws to address the privacy concerns of innocent people whose information has been swept up in these programs, and to rebuild confidence in the US technology industry.”

Zoe Lofgren, a Democratic congresswoman whose California district includes part of Silicon Valley, said it was too early to determine the extent to which the intervention would alter the debate in a significant way. She pointed out that while the tech industry has a large and growing influence in Washington, and includes major donors to Obama’s re-election campaign, “they’ve not always, or even often, got their way on policy issues”.

“It is my perception that the spy agencies are pretty much oblivious to the impact on the country when it comes to the economy, credibility and the like,” she told the Guardian. “I don’t know if this is going to convince them.”

Lofgren is close to the tech industry; earlier this year she introduced legislation to enable tech firms to disclose the nature of government surveillance orders they receive – a proposal that was incorporated into the USA Freedom Act and was among the demands in Monday’s open letter.

She said that irrespective of whether Congress implemented the kind of reform advocated by the likes of Google, Yahoo and Microsoft, those companies already recognise “the potential economic harm to overseas business is substantial”, and are taking defensive action. That involves hiring the best cryptologists they can find in what Lofgren described as a developing “encryption race” between the private sector and government.

“All of these companies are scrambling to increase security in order to prevent surveillance from their own government,” she said. “It is very clear that the technology companies are not going to wait for Congress. They are going to amplify and deploy additional measures to protect the privacy of their users.”

Additional reporting by Spencer Ackerman in New York