Benjamin Pell, 35, took black rubbish bags from outside the Harley Street offices of Harkavys at night, disguised as a Westminster council cleaner.
Blackfriars Crown Court was told that among the items found later in Pell's possession were documents revealing that Aitken was embroiled in "personal and public difficulties". Edward Brown, for the prosecution, said the probability was that Pell intended to sell the revelations to a publisher.
The court heard that Pell's home in Hendon, north London, was full of black bags containing similar information. But Eamon Sherry, for the defence, told the court that Pell had an obsession with collecting rubbish - a debilitating and untreatable illness.
The defendant was caught because staff at Harkavys became suspicious when rubbish left out for collection was interfered with on several nights in February. The police set up a surveillance operation over the offices on 26 February.
Mr Brown said: "The defendant was seen going towards 13 Harley Street with some black bags. He walked down to the basement, where he was seen to swap his bags for the ones left out for the bin men. He was returning to his vehicle with four bags when police stopped him."
Police searched Pell's home later that day and discovered between 100,000 and 200,000 documents, as well as a copy of a High Court judgment telling the defendant "that he should not go about collecting confidential waste", Mr Brown said.
Pell admitted a charge of theft from Harkavys Solicitors on 26 February. Sentencing was deferred to 10 December.
Pell was granted bail on condition that he resided at his home address and kept a night-time curfew.