The public inquiry into undercover policing aims to "get to the truth" and make known the full facts about tactics used by police spies over decades, the hearing has been told.

In his opening statement delivered via a live video stream on Monday, counsel to the inquiry David Barr QC set out the background to the investigation and why it was established in 2015.

In 2014 it emerged that undercover officers spied on the Lawrence family's campaign for justice following the notorious 1993 racist murder in Eltham.

He said: "This inquiry has been set up as a result of profound and wide-ranging concerns arising from the activities of two undercover police units.

"First, the Special Demonstration Squad (SDS), which existed between 1968 and 2008, second, the undercover element of the National Public Order Intelligence Unit (NPOIU), which existed between 1999 and 2010.

"It has emerged that for decades undercover police officers infiltrated a significant number of political and other activist groups, in deployments which typically lasted for years.

"The information reported by these undercover police officers was extensive. It covered the activities of the groups in question, and their members. It also extended to the groups and individuals with whom they came into contact, including elected representatives.

"Reporting covered not only the political or campaigning activities of those concerned but other aspects of their personal lives.

"Groups mainly on the far left but also the far right of the political spectrum were infiltrated, as well as groups campaigning for social, environmental or other change."

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Then-home secretary Theresa May set up the inquiry into undercover policing in England and Wales in 2015 after condemnation of the tactics used by the two secret units.

"The inquiry will be seeking out the truth," Mr Barr said. "Publicly wherever that is possible, so that the full facts become known and appropriate recommendations can be made for the future conduct of undercover policing."

Methods employed by the police spies included using the names of dead children as cover identities without their families' consent.

Mr Barr said: "We will be receiving evidence that a number of the undercover officers who served with the SDS and the NPOIU engaged in sexual activity in their cover identities.

"Several formed long-term sexual relationships; in some cases the officer did eventually reveal their cover identity, in other cases they did not do so.

"At least one fathered a child with a woman who did not know that her partner was an undercover police officer. In many cases, deception has had devastating consequences."

Long-term deception also took its toll on the officers, Mr Barr said.

"The impact of conducting long-term undercover operations of the sort conducted by the SDS and the NPOIU on the mental health of some undercover officers appears to have been considerable.

"In some cases, particularly those in which the undercover officer has been involved in a long-term deceitful sexual relationship, the officer's family has also suffered."

Members of trade unions were blacklisted from work after the groups were infiltrated by police, and family justice campaigns.

There were also miscarriages of justice when undercover officers failed to reveal their true identities in criminal court proceedings, the inquiry was told.

The mammoth investigation is being heard in tranches by date.

The hearings planned for November will include opening statements by core participants, followed by evidence on the activities of the SDS between 1968 and 1972.