The deal forged by the then Foreign Minister Pierre Graber saw the Alpine country providing diplomatic support to the Palestinian Liberation Organization. In exchange, Switzerland would be spared from terror attacks, according to research carried out by Marcel Gyr, a journalist with Swiss national daily the Neue Zürcher Zeitung.

The deal – allegedly so secret that the other members of Switzerland’s seven-member government did not even know about it – came at a time of high tension and in the context of several terror attacks.

In February 1970, a Swissair plane crashed just after taking off from Zurich’s Kloten airport, killing all 47 passengers, with Palestinian terrorists believed to be behind the attack, although no official link was ever established

Later that year, Palestinian commandos hijacked a Swissair plane bound for New York and diverted it to Jordan, with 157 passengers held for a week.

But Switzerland’s decision to go it alone in dealing with the PLO put in on a potential collision course with foreign powers including the US and the UK as the country was officially part of a united front against Palestinian terrorism, Gyr argues in his book.

Switzerland also risked its reputation as a neutral state by carrying out the negotiations, NZZ reports.

Although the Diplomatic Documents of Switzerland research centre has downplayed the role of those talks, revelations about the deal – details of which are still a state secret under Swiss law – are big news in Switzerland.

This is partly because the negotiations were purportedly carried out by a man the NZZ calls the ‘enfant terrible’ of Swiss politics, Jean Ziegler. Ziegler, a former politician and sociology professor at the University of Geneva, is a renowned critic of Swiss international policy.

Speaking about the deal between Switzerland and the PLO, Ziegler told Swiss public broadcaster SRF his role in the negotiations had been “very modest” and he had played no part in the decision-making process.

But he went on to describe the deal as “immoral” and shocking”.

Graber “acted for reasons of State – the protection of the Swiss population from new attacks” and by doing so he had caused harm to the concept of the “the rule of law”, the 82-year-old said.