BAE Systems paid a company £2,500 a month to monitor events involving the firm, including what was being said about it, according to court documents.
Paul Mercer, a researcher for the company LigneDeux, said in a sworn affidavit that he was sent an anonymous package containing data, including an email which had details of a judicial review by Campaign Against Arms Trade (CAAT).
The review is being brought by CAAT against the Serious Fraud Office and is related to the dropped investigation into an arms contract between BAE and Saudi Arabia.
CAAT then took BAE to court in January to force the disclosure of how BAE received the email detailing the group's legal advice.
Mr Mercer said in his affidavit that he did not know who had sent him the package and passed on the email to BAE's head of security Michael McGinty.
According to the court documents, Mr Mercer said that Mr McGinty told him he should not send any more unsolicited material and told him two months later that he had been named as the source of the email.
CAAT told the Daily Telegraph: "We believe the actions of Mr Mercer and BAE raise issues of the utmost public importance."
BAE said it was preparing a statement on the matter.
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