An inquiry has been launched into allegations the Metropolitan Police shredded documents months after a probe into undercover policing began.
The Independent Police Complaints Commission (IPCC) said it is looking into claims the Met's National Domestic Extremism and Disorder Intelligence Unit destroyed paperwork in May 2014.
The public inquiry into undercover policing was announced in March 2015
The Met said it is committed to co-operating with the investigation.
The IPCC said documents should not have been destroyed "without express permission".
IPCC spokeswoman Sarah Green said: "While the evidence indicates that a large number of documents were shredded over a period of days in May 2014, the difficult task ahead for our investigators is to determine what the documentation was, why it was destroyed, whether electronic copies were kept and who may have ordered its destruction."
The then Home Secretary Theresa May ordered the public inquiry following a number of accusations about the conduct of officers deployed to spy on political groups.
The Met said in a statement it had notified the IPCC about the possible shredding of documents by the National Domestic Extremism and Disorder Intelligence Unit in May 2016, but the allegation was not recorded until December 2016.
"The reason why it was not progressed further at that stage is unclear and will also be subject of the investigation by the IPCC," the statement said.
"The MPS has briefed the Public Inquiry into Undercover Policing regarding the referral."
Green Party member Baroness Jenny Jones has alleged records relating to her were destroyed or deleted in June 2014.
Her case is "now also subject to independent investigation" and is a separate matter from the May 2014 documents, the IPCC said.