ANYONE with evidence of government corruption or warmongering should come forward "before it's too late", several high-profile whistleblowers said yesterday.

Fifteen years ago today, the Observer revealed a US "dirty tricks" campaign to secure backing for the invasion of Iraq (which the paper nonetheless supported).

A leaked memo revealed surveillance operations by the US National Security Agency (NSA) and GCHQ, involving intercepting the telephone calls and emails of UN delegates before a crucial security council vote.

The objective, the memo revealed, was to obtain "information that could give US policymakers an edge in obtaining results favourable to US goals."

Former GCHQ linguist and analyst Katharine Gun leaked the memo to the Observer soon after she received it on January 31, 2003.

Her prosecution under the Official Secrets Act eventually collapsed in February 2004.

She said her response to receiving the memo was “almost instantaneous,” comparing it to seeing “a child unknowingly wander into the middle of a road – our instinctive reaction would be to intervene.”

But she said she was disappointed with the reaction as it “didn't get the level of attention it deserved.”

Former US Marine Matthew Hoh, who resigned from the State Department in protest at the war in Afghanistan, told potential whistleblowers: “Listen to your conscience and act before it's too late … because of what will happen to your soul and your mind if you don't act.”

Ms Gun warned potential whistleblowers and journalists about the “draconian” Official Secrets Act, which was used in pursuit of the “criminalisation of whistleblowers and journalists.”

Ms Gun spoke about Liberty's challenge to the Investigatory Powers Act earlier this week, warning: "Once governments gain that level of power, they don't relinquish it – they just build on it."