Swiss police foiled a suspected espionage operation in which Russians posed as plumbers to spy on the Davos summit, according to a report.

Two Russian men were intercepted by authorities in the Alpine resort in August last year, five months before the town hosted this week’s World Economic Forum (WEF.)

Police in the eastern Swiss canton of Grisons confirmed the men had been subject to routine identify checks and were found to have Russian diplomatic passports.

“We could not ascertain any reason to detain them. They were allowed to go,” a cantonal police spokeswoman said.

She did not confirm the men posed as plumbers and said officers had not established a link between their visit and WEF, where American president Donald Trump and prominent Kremlin critic Bill Browder are among attendees.

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But a detailed report in the Swiss national newspaper Tages-Anzeiger said the Russians were suspected of carrying out “preparatory work for spying on the World Economic Forum”.

The men had aroused the suspicion of local police because they were booked to stay in the high-end resort for three weeks, according to the Zurich-based publication, which cited several anonymous sources.

The sources said it was “obvious” the Russians “had their sights on the WEF” and might have planned to bug or hack the summit.

The men claimed diplomatic protections but had not been accredited as diplomats by Bern, police confirmed.

Moscow reportedly threatened diplomatic consequences if its men were arrested, but despite Grisons police alerting Switzerland’s intelligence services the Russians were allowed to leave as they insufficient evidence to prove they planned to spy on Davos.

A spokesman for the Russian embassy in Bern dismissed the report, saying two Russian diplomats accredited outside Switzerland had been checked and allowed to go on their way.

“Diplomatic passports are given to high-ranking officials, not to manual labourers,” he said. “I think this was probably a dumb joke.”

Maria Zakharova, a spokeswoman for the Russian Foreign Ministry in Moscow, said she was not aware of the incident.

Mr Browder, an investment fund manager who has campaigned to expose Kremlin corruption, said the alleged espionage operation showed the reach of the Russian state.

“The Russians are actively targeting all of their enemies in all different countries. They have huge resources and Davos is an important place, and this is the one place I can come and personally challenge Russian officials over the murder of Sergei Magnitsky,” he added.