Amnesty International accuses Shell of false claims about its environmental impact in the Niger delta, saying that the oil company cannot be trusted and that there are "serious discrepancies" between the evidence of pollution and what Shell claims.

"[Its] claims about its environmental impact in the Niger delta are frequently untrue. Shell has claimed that the oil spill investigations are sound when they are not, that sites are cleaned up when they are not, and that the company is transparent when, in reality, it maintains very tight control over every piece of information – deciding what to disclose and what to withhold," a report into oil spills in the Nigerian region says. "Shell is being disingenuous about the devastation caused by its Niger delta operations. Shell's claims about the oil spills cannot be trusted."

According to official figures, there are several hundred oil spills a year in the delta, many of which involve Shell pipelines. "Instead of being in the dock when there is an oil spill in Nigeria, Shell gets to act as judge and jury," says the Nobel prize-winning human rights organisation. "It is the communities that suffer a life sentence, with their land and livelihoods destroyed by the pollution. The Niger delta is the only place in the world where companies brazenly admit to massive oil pollution from their operations and claim it is not their fault. Almost anywhere else they would be challenged on why they have done so little to prevent it."

The report argues that the investigations oil companies must conduct into spills are seriously flawed. "So-called official investigation reports into the cause of oil spills in the Niger delta can be very subjective, misleading and downright false. This is a system that is wide open to abuse – and abuse happens. There is no one to challenge the oil companies and almost no way to independently verify what they say. In effect it's 'trust us – we're big oil'," said Audrey Gaughran, director of global thematic issues at Amnesty and lead researcher on the report.

The report, which was shown to Shell before publication and includes the company's denials, refutations and explanations, is likely to be explosive in Nigeria, where the company is a major employer and one of the biggest generators of foreign currency.

A Shell spokesman in London said: "The Shell Petroleum Development Company of Nigeria (SPDC) firmly rejects unsubstantiated assertions that they have exaggerated the impact of crude oil theft and sabotage to distract attention from operational performance. We seek to bring greater transparency and independent oversight to the issue of oil spills, and will continue to find ways to enhance this. These efforts include publishing spill data online since January 2011 and working with Bureau Veritas, an independent third party, to find ways to improve the immediate response to a spill. It must be emphasised that the joint investigation process is a federal process that SPDC cannot unilaterally change, involving as it does representatives of regulatory bodies, the ministry of environment, the Nigerian police force, state government and impacted communities."

The company called on the Nigerian government and civil society to end the theft of oil, which regularly forces its pipelines to close. "Solutions to the terrible tragedy of oil pollution in the Niger delta need to be found. Crude theft continues to affect people, the environment and the economy. Co-ordinated action from the industry, government, security forces, civil society and others is needed to end this criminality. SPDC regrets that some NGOs continue to take a campaigning approach rather than focusing on on-the-ground solutions that bring societal benefits."

The report voices particular concern about Shell's activities in Ogoniland, the 400-sq-mile region in the delta whose people led a revolt against Shell's pollution in the 1990s and forced the company to withdraw. "When Shell left Ogoniland it did not properly decommission its facilities, leaving them open to interference, and communities exposed to the associated risks," it says. "This is completely contrary to international oil industry standards as well as international standards on business and human rights, both of which require that Shell exercise adequate due diligence in relation to prevention of sabotage, theft and the associated human rights and environmental risks.

"Shell has claimed that the reason that it never properly decommissioned its Ogoniland facilities and made them safe over the last 18 years was lack of access. This is not the case. Shell has had access to Ogoniland over the last 18 years, including to carry out the highly inadequate clean-ups that Unep [the UN Environment Programme] documented. Shell's access to Ogoniland is undoubtedly restricted at times, but Shell cannot defend its failure to properly decommission facilities in Ogoniland over 18 years by reference to problems of access."

Amnesty International and the Centre for Environment, Human Rights and Development in Nigeria claim to have found evidence of Shell having changed the officially recorded cause of a spill after an investigation had taken place at Bodo. In one incident, secretly filmed video of an investigation shows how officials from Shell and the regulator tried to subvert the evidence by persuading community members on the investigation team not to attribute the cause to equipment failure.

Footage of an oil leak in Bodo from 2008 reviewed by a US spill-monitoring company suggests that Shell seriously under-recorded the volume shed. "Shell's official investigation report claims only 1,640 barrels of oil were spilt in total, but other evidence points to the amount being at least 60 times higher. Shell has repeatedly claimed to its investors, customers and the media that sabotage and theft were behind the vast majority of spills – but the facts do not support this assertion," the report says.