A review has cleared the scientist Peter Gleick of forging any documents in his expose of the rightwing Heartland Institute's strategy and finances, the Guardian has learned.

Gleick's sting on Heartland brought unwelcome scrutiny to the organisation's efforts to block action on climate change, and prompted a walk-out of corporate donors that has created uncertainty about its financial future.

Gleick, founder of the Pacific Institute and a well-regarded water expert, admitted and apologised for using deception to obtain internal Heartland documents last February.

He has been on leave from the institute pending an external investigation into the unauthorised release of the documents, although it is not entirely clear what the investigation entailed. That investigation is now complete, and the conclusions will be made public.

It was not immediately clear the findings would allow Gleick to make an early return to his job at the Pacific Institute. However, despite the official leave, Gleick has remained professionally active, appearing at public events and accepting speaking engagements. He delivered an Oxford Amnesty lecture on water in April.