Amnesty International has accused major oil companies, including Shell, of failing to report the true picture of oil spills in Nigeria.
Amnesty says oil companies often blame oil spills on sabotage in order to get out of paying compensation when in fact corroded pipes are the cause.
The report says the process of cleaning up the oil spills is completely discredited.
Shell said it "firmly rejects unsubstantiated assertions".
It highlighted the issue of theft of crude oil, which it said "remains the main cause of oil pollution in the Delta".
Oil spills are having an appalling environmental impact on the Niger Delta and they are happening at an alarming rate, says the BBC's Will Ross in Lagos.
In its report, Amnesty identifies a "staggering" 474 spills in 2012 in one area alone, operated by the Nigerian Agip Oil Company - a subsidiary of Italian firm ENI.
Agip's head of operations in Nigeria, Ciro Pagano, told the BBC's Newsday programme that all the spills were recorded so there was little room for dispute.
He also said that Eni paid all compensation due to local communities, according to Nigerian law.
Working with a local human rights group, Amnesty studied the oil spill investigation process in Nigeria over six months.
It claims there is "no legitimate basis" for the oil companies' claims that the vast majority of spills are caused by sabotage and theft.
Members of the local community together with oil company staff and government officials are supposed to investigate oil spills, but Amnesty calls this Joint Investigation Visit (JIV) process "wholly unreliable" because, it says, the companies themselves are the primary investigators and the process lacks transparency.
It says this means that both the causes and severity of oil spills may therefore be misrecorded, sometimes meaning affected communities miss out on compensation.
"Sabotage and theft of oil are serious problems in the Niger Delta," the report acknowledges.
"However, international oil companies are overstating the case in an effort to deflect attention away from the many oil spills that are due to corrosion and equipment failure. Moreover, securing oil infrastructure against such acts is - to a substantial extent - the responsibility of the operator."
It says the majority of the report's findings relate to Shell as the primary operator in the Niger Delta - though it acknowledges improvements to Shell's JIV process since 2011.
'Criminality to blame'
It also points out that Nigerian Agip suffered more than double the number of spills as Shell, though it operates over a smaller area.
Mr Pagano accepted that it was a "very serious, complex problem" and called for all stakeholders to work together to solve it.
Shell said it "firmly rejects" the claims.
"We seek to bring greater transparency and independent oversight to the issue of oil spills, and will continue to find ways to enhance this."
It said the JIV process was a federal process the company could not unilaterally change.
Stolen oil, Shell said, costs Nigeria billions of dollars in lost revenue.
"Co-ordinated action from the industry, government, security forces, civil society and others is needed to end this criminality, which remains the main cause of oil pollution in the Delta today," Shell said.
It said it regretted "that some NGOs continue to take a campaigning approach rather than focusing on on-the-ground solutions that bring societal benefits".