Intercepting the telephone calls of Angela Merkel would have been "politically unwise" and "certainly illegal under German law", according to a former senior British secret intelligence officer.

However, he says that under international law, tapping into the German chancellor's telephone conversations "would appear to be fair game".

Nigel Inkster, former deputy chief of MI6, was responding to the disclosure by Edward Snowden that the US National Security Agency targeted Merkel's mobile telephone. Though the White House has not officially admitted it, it has said the US will not monitor the chancellor's conversations in future.

Writing in the latest edition of Survival, the journal of the International Institute for Strategic Studies, where he is director of transnational threats and political risk, Inkster says: "Listening to chancellor Angela Merkel's mobile telephone calls, as the US has been accused of doing, might be judged to be politically unwise and was certainly illegal under German law. But, under international law, her telephone conversations would appear to be fair game."

In her latest weekly podcast, Merkel suggested that a European communications network should be set up to offer protection from surveillance by the NSA and its British eavesdropping partner, GCHQ. She plans to discuss the idea with the French president, François Hollande, when she meets him in Paris on Wednesday.

"Above all we'll talk about European providers that offer security to our citizens, so that one shouldn't have to send emails and other information across the Atlantic," she said in the podcast. "Rather, one could build up a communications network inside Europe."

Inkster says that while NSA surveillance programmes violated the domestic laws of countries subjected to espionage, it was less obvious that they violated international law.

"International lawyers hold a wide spectrum of opinions on the legality of espionage, and there was nothing in existing international law that expressly proscribed espionage," he writes.

He says it is clear that the NSA's own protective security was "not fit for purpose" in dealing with what he calls an "insider threat" – a reference to Snowden, a former NSA contractor. That has long been recognised.

But Inkster claims there was an implicit argument by newspapers publishing the Snowden revelations that the data was "so promiscuously distributed" that the NSA deserved to have its secrets exposed. That argument is self-serving and does not stand up to close analysis, says Inkster.

In his article, titled The Snowden Revelations: Myths and Misapprehensions, he says the NSA had no interest in the "private communications of ordinary citizens" in the US or elsewhere, and lacked the motivation and resources to monitor them "on a systematic or intensive basis". The term "mass surveillance" is a misnomer, says Inkster.

He says Snowden's revelations will undoubtedly act as a catalyst for some states to speed up their efforts to diversify their communications networks "to minimise dependence on US systems".

This is an issue that Merkel said she would raise with Hollande in Paris. But the process of avoiding having to rely on US networks "began some time ago", writes Inkster.

In response to a question in September about the impact of Snowden's disclosures, Inkster said he sensed that "those most interested in the activities of the NSA and GCHQ have not been told very much they didn't know already or could have inferred".

Inkster says in his Survival article that it is likely that some members of the British cabinet were not made privy to the details to GCHQ's "collection programes", because "they would not need this knowledge to perform their functions". Those cabinet members who did need to know "would have been appropriately briefed", he says.