Trial : Marcin Kostrzewa ( pictured today arriving at Plymouth Crown Court ) allegedly broke into his neighbour 's flat and took the restricted files . A Polish man who tried to sell secret nuclear submarine documents he stole from his neighbour 's airing cupboard has been jailed for more than four years . Marcin Kostrzewa was sentenced to four-and-a-half years behind bars after a jury at Plymouth Crown Court took less than an hour to find him guilty following a three day trial . The court had heard how Kostrzewa broke into neighbour Shane Spencer 's flat and took the restricted files . He had become 'obsessed ' with Mr Spencer after finding out he worked with subs . Marcin later stole the paperwork after using English and Polish words to search Google using terms like 'spy games , spies and secret documents ' . Kostrzewa then contacted the Polish embassy and tried to sell the information for £50,000 but was trapped in an undercover sting . He had denied committing the burglary between January and April 2014 and claimed he had found the contents in a bin as he was carrying out research for a spy novel . The 31-year-old claims he just wanted to get arrested so he could meet a real MI5 Agent . But jailing him for four and a half years , Recorder John Williams said : 'You are not the bumbling fool you have sought to portray in your evidence . 'In my view you are both calculating and clever . 'You stole and sought to sell documents which you understood to be sufficiently sensitive and no doubt secret to warrant a large amount of money . ' He added that the loss of the documents could 'affect military operations ' - though the metalworker tried to sell them to a friendly government . In a statement read to court , Mr Spencer said he had been suspended from his work at Devonport Dockyard for Babcock Marine . Aerial view : The jury heard at the time Mr Spencer was working as an engineer at Devonport ( above ) and was involved with designs for Trafalgar and Astute class submarines . The court heard he had lost his high level of security clearance and was thinking of moving abroad . Alexander Chalk , for the Crown Prosecution Service , said the Pole had previous convictions in his home country , including robbery and burglary . He added the documents were marked restricted , the third of five classifications at the time , below top secret and secret .
Marcin Kostrzewa , 31 , took restricted files from flat next-door . Became 'infatuated ' with Shane Spencer after finding out about his work . - In the summer of 2014, he was arrested for possession and distribution of child pornography on an iPad. Jailed for four-and-a-half years after jury finds him guilty of burglary .