A Dutch court has sentenced a diplomat to 12 years in prison for delivering confidential Nato and European Union documents to Russian agents.

The case, one of the worst spying affairs in the Netherlands in memory, is linked to that of a couple on trial in Germany on similar charges.

A three-judge panel at The Hague district court on Tuesday said Raymond Poeteray had damaged the interests of the Dutch state and its allies by passing on sensitive military and political documents.

"The court considers it proven that he passed on confidential documents to the Russian Federation for years and on assignment from the Russian foreign intelligence service," the presiding judge AJ Milius said, reading a written summary of the panel's ruling.

He said Poeteray gave the Russians information about the civil war in Libya, EU fact-finding missions in Georgia, and Dutch peacekeeping missions in Kosovo and Afghanistan, causing substantial damage.

"In addition, [Poeteray] did not shy away from giving away confidential and very private personal information about seven colleagues," potentially making them vulnerable to blackmail, the judge said.

Poeteray, 61, was working at the Netherlands' foreign affairs ministry after serving in the Dutch embassies in Hong Kong and Indonesia earlier in his diplomatic career, which began in 1978. He was arrested in March 2012 in connection with a German investigation.

In Germany, a couple who called themselves Andreas and Heidrun Anschlag – their true identities are unknown – were arrested in October 2011. They are on trial for allegedly compiling the information that Poeteray gathered and sending it on to Russia's intelligence agency.

The Hague court found Poeteray "acted purely in pursuit of profit, to pay off his debts and to be able to allow himself a certain lifestyle".

He was paid at least €72,000 (£61,000) in cash alone between January 2009 and August 2011, the ruling said.

Poeteray's lawyers had argued he was innocent. He explained the large amounts of cash in his bank accounts as profits from the sale of jewellery that he had cheaply bought while serving overseas.

Judges rejected that as not credible, given that he was deeply in debt. They also noted that instructions Poeteray had received via the Anschlags had told him to use that explanation.

Defence and prosecution lawyers have two weeks to consider whether they will appeal. Prosecutors had asked for a 15-year sentence.

The court granted one request by Poeteray: it ordered the return of five of his watches seized during the investigation, branded as an Omega, a Graham, a Breitling, a Bulgari, and a Corum. All were fakes.