A former head of the Royal Navy has branded Edward Snowden a "traitor" and "irresponsible" after the US whistleblower declined to reveal if he had read every document he leaked to the press.

Lord West, a former First Sea Lord and security minister, said the ex-National Security Agency (NSA) worker had made the UK public "less safe" as a result of the revelations he made about government-level monitoring of online communications.

His comments come after Snowden appeared on US television show Last Week Tonight with John Oliver, a British comedian, and was pressed on whether he had read each of the thousands of files he released.

When asked "how many of those documents have you actually read?" Snowden replied: "I've evaluated all the documents that are in the archive."

As Oliver pressed him, he said "I do understand what I turned over," before admitting "I recognise the concern" about his and the media's competency in handling the documents.

Oliver cited one example in which the New York Times published a slide with sensitive information relating to al Qaida, which Snowden admitted was a "f*** up", but defended journalists, saying many had taken "extraordinary" measures to be responsible with the data.

But Labour peer Lord West told the Daily Mail: "Since the revelations of the traitor Snowden, terrorist groups have changed how they communicate and talk to each other.

"His actions have made us all less safe. No doubt people will die who would not have died had he not been so irresponsible."

Robin Simcox of the Henry Jackson Society told the paper: "This interview seemed to show how little Snowden had thought about the potentially deadly consequences. Snowden stole a huge amount of sensitive documents an as a result terrorists and other serious criminals have adapted their methods accordingly."

Snowden was a contractor working in Hawaii for the NSA when he became uncomfortable with the level of intrusion the agency and its foreign counterparts, including Britain's GCHQ, were making into the lives of innocent civilians.

After raising the alarm with his superiors, he decided to blow the whistle and stole tens of thousands of classified files with the intention of showing them to the world.

He fled to Hong Kong where he met with journalists working for The Guardian, as well as documentary film-maker Laura Poitras, to co-ordinate a series of articles that would expose mass surveillance programmes such as the NSA's Prism and GCHQ's Tempora, which involve "hoovering up" vast volumes of private communications.

Once Snowden's identity was out, he fled to Russia, where he remains wanted by the US authorities.