A senior official in former Ontario premier Dalton McGuinty's office instructed staff to "double delete" e-mails, to ensure that nothing was turned over to a legislative committee probing the scrapping of two gas-fired power plants, a criminal trial was told.

David Livingston, Mr. McGuinty's former chief of staff, and his deputy Laura Miller are facing criminal breach of trust and mischief charges in connection with the destruction of e-mails and other government records related to the controversial cancellation of the two power plants prior to the 2011 provincial election. They have each pleaded not guilty.

The purging of documents while the McGuinty government was under "intense public pressure" to produce records forms the backdrop to the trial, prosecutor Sarah Egan said in her opening remarks at Toronto's Old City Hall court building on Friday. She noted that a legislative committee had ordered the government to release all documents related to the decision to pull the plug on the two power plants.

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"This trial is not about the inadvertent failure to preserve documents but the intentional destruction of data for the purpose of thwarting the public's right to accountability and transparency," Ms. Egan said in the Ontario Court of Justice.

"Mr. Livingston instructed his staff on how to double delete e-mails," Ms. Egan alleged, "that is, how to delete e-mails permanently so they could not be recovered later on."

The charges stem from police accusations that Mr. Livingston hired a non-government IT expert, Ms. Miller's spouse, Peter Faist, to "wipe clean" computer hard drives in the premier's office just days before Mr. McGuinty left office in February, 2013. The two are accused of compiling a list of senior Liberal staffers whose computer records were to be deleted, all of whom police allege were involved in discussions around the Liberals' costly decision to scrap the gas-plant projects in Mississauga and Oakville, Ont. The provincial auditor pegs the cancellation tab at $1-billion.

Mr. McGuinty, who is not under investigation and who is co-operating with the probe, has told police his chief of staff was ultimately responsible for all of the activities that took place in his office.

Court documents prepared by the Ontario Provincial Police as part of their investigation say Mr. Livingston avoided leaving public records of many of his discussions by double deleting e-mails and communicating by BlackBerry Messenger. Mr. Livingston was also part of a high-level staff initiative in the premier's office, codenamed Project Vapour, to manage the fallout from the cancelled Oakville power plant.

E-mails on Project Vapour were contained in thousands of pages of documents made public in October, 2012, just days before Mr. McGuinty announced his resignation and asked the lieutenant-governor to prorogue the legislature. All business ground to a halt, including committee hearings into the cancelled power plants.

Mr. Faist, who is co-operating with investigators, told police he was instructed by Ms. Miller to "wipe off personal data" on about 20 computers in the premier's office.

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In her opening remarks, Ms. Egan said Mr. Livingston was cautioned on Jan. 31, 2013, by Dave Nicholl, the province's corporate chief information officer, to preserve e-mail accounts and records related to the power plants. The following week, she said, Ms. Miller's spouse began wiping the hard drives.

Much of Friday's hearing was devoted to prosecutors seeking to have former OPP detective sergeant Robert Gagnon qualify as an expert witness in the operation of computer systems and the forensic recovery and interpretation of computer data. Mr. Gagnon, who retired from the OPP in 2009, was retained by his former employer to help with the investigation, known internally as Project Hampton.

Lawyers for the defence, however, are seeking to exclude Mr. Gagnon as an expert witness, citing what they say is a significant role he played in shaping the investigation. Scott Hutchison, Ms. Miller's lawyer, told the court that it was Mr. Gagnon who proposed that his client and Mr. Livingston could also be charged with mischief in addition to breach of trust.

Justice Timothy Lipson plans to rule on the matter on Wednesday. The trial continues on Monday.