Portugal’s intelligence service is hunting for possible accomplices of an alleged double agent who was caught passing Nato secrets to a Russian handler.

Frederico Carvalhão, a senior figure in Portugal’s SIS intelligence service – the country’s version of MI5 – was arrested in Rome last Saturday, allegedly in the act of passing secret documents to a supposed Russian agent.

But there are suspicions that Mr Carvalhão was not working alone and there could be other moles within SIS. Portugal is a member of both Nato and the European Union, giving its security services access to some of the secrets of the two organisations, both of which are key targets for Russian intelligence.

Mr Carvalhão was detained by Italian police at a café in Rome’s Trastevere district after a sting operation coordinated with Portuguese agents. Also seized was a Russian citizen, believed to be an officer from the SVR foreign intelligence service.

Augusto Santos Silva, the Portuguese foreign minister, confirmed on Wednesday that there was “an ongoing judicial investigation” into the case. The focus is believed to be on discovering who helped Mr Carvalhão extract top-secret documents from Ameixoeira Fort, the SIS headquarters in Lisbon.

The use of USB sticks is prohibited inside Ameixoeira, access to all printed documents is registered and those classified as secret have an invisible watermark to allow for the detection of anyone who has removed them. Yet Mr Carvalhão was allegedly caught while handing over six "top secret" documents covering Nato defence systems, the communication infrastructure between member countries and military bases.