The United Nations Environment Programme (UNEP) is preparing to issue a report announcing that 90% of the oil spills in Ogoniland, Nigeria, are caused by the locals stealing crude from pipelines – and that Shell's aged pipelines and ill maintained installations account for a mere 10% of the spills. Why so little, we might ask?

The UNEP has now admitted this figure is based on data from the oil industry and the Nigerian government. It's not surprising that this is in line with what Shell used to claim in the 1980s – that about 80% of the oil spills were caused by vandalism or sabotage. This claim that infrastructure has been sabotaged is particularly attractive to oil companies, because they are then exempted from paying compensation for any resulting spills. Why accept responsibility for polluting the locals' creeks, swamps and farmlands and destroying their livelihoods when you can blame the very same people for the mess now coating their own backyards with a toxic gloss?

Yet crucial expertise which could have painted a very different picture was sidelined. Prof Richard Steiner, an international expert on oil spills, was contracted to write the manual on oil damage assessment and restoration by the UNEP in 2004. But when Shell hired the agency to carry out the present study, Steiner's offer to provide scientific advice and guidance to the Ogoniland report was declined.

Steiner has already said that the findings now uncovered are incorrect, and has gone on to say: "Our earlier results suggest that much of the oil spilled there was due to poor practice by Shell, rather than bunkering and sabotage… it is entirely implausible that 90% of the oil spilled was due to bunkering [the act of criminal gangs stealing oil]." In short, his opinion is that this is not an independent, credible assessment.

The report does indeed rely heavily on figures produced by oil companies and Nigerian state statistics rather than on testimonies from those most affected – the communities in Ogoniland.

The National Oil Spill Detection and Response Agency of Nigeria has reported a total of 3,203 oil spills in the Niger Delta region in the last four years alone.

That list lengthens every day. The records of the Nigerian Directorate of Petroleum Resources show that nearly 2.5m barrels of crude oil were spilt between 1976 and 1996. Most damning of all, 77% of this oil was not recovered and contaminated the local environment. This is an environmental catastrophe which has a long history – some notable past spills include the Escravos spill of 1978 in which 300,000 barrels of crude oil was spilled into the coastal waters and another, in the same year, caused by tank failure at Forcados Terminal in which 580,000 barrels were spewed into the environment.

It is in this polluted environment that the people of Ogoniland have had to live for decades with spill after spill. The UNEP must be, and be seen to be, an independent arbiter of what has really happened there. There should be no room for suspicion that the $10m (£6.5m) Shell paid the agency for this report will influence the outcome.

• Nnimmo Bassey is chairman of Friends of the Earth International