Shell's persistent claims that theft and sabotage are responsible for almost all the oil spills at its operation in Nigeria could have been exaggerated, an international watchdog has found.

The National Contact Point (NCP) agency will today give Shell a rap on the knuckles for its reporting of its spills in the Niger Delta region, some of which have been highly damaging to the environment.

The agency, set up to oversee OECD guidelines on multi-national companies, said: "Given the many years of discussion about the causes of oil spills in Nigeria, Royal Dutch Shell management should have had a more cautious attitude about the percentage of oil spills caused by the sabotage," noting that the data they are based on is "not absolute".

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It called on Shell to "be prudent with regard to general communication to stakeholders of very detailed figures on oil spills, when discrepancies exist with regard to the causes or amounts".

NCP has reached this conclusion following an investigation into accusations by Amnesty International and Friends of the Earth International that Shell has exaggerated the proportion of spills caused by sabotage to avoid paying compensation and to reduce damage to its reputation.

Amnesty and Friends of the Earth gave a qualified welcome to the NCP report, arguing that it did not properly hold Shell to account. They said it did not rule on whether Shell had breached OECD guidelines, failed to investigate all the evidence they had presented to the agency, and did not look into whether Shell's statements were misleading.

Paul de Clerck, of Friends of the Earth Europe, said: "Today the NCP failed to speak out against Shell's abuse in Nigeria. It did not access key evidence provided and thereby let the company off the hook." The organisations said they provided evidence of "serious flaws" in the system used by Shell for investigating spills, including video footage of one investigation which highlighted several serious shortcomings in its procedures.

NCP acknowledged that the process of checking the origin of spills in Nigeria relies on information from the company itself that is not verified by any independent bodies. It cited a finding from the UN Environment Programme in 2011 that "government agencies are at the mercy of the oil companies when it comes to conducting site inspections".

Shell's Nigerian operation, SPDC, is run by Shell, although it has only a 30 per cent stake in the group, which is majority-owned by the country's government. It has increasingly blamed spills on oil theft and sabotage, which it claims accounted for 95 per cent of spills by volume last year and 80 per cent by number.

A Shell spokesman said: "SPDC has been found to be in compliance with the OECD guidelines for multi-national enterprises. SPDC operates in a way that is consistent with the OECD guidelines and is open to working with others that want to help address the challenges faced in Nigeria." He added that there is no issue in the NCP's report over any of Shell's "current data" in relation to the cause of spills. He also pointed to NCP's recommendation that Amnesty and Friends of the Earth "be prudent with regard to publications or campaigning".