The Government has been warned by MPs not to rush into a new system which could give developers “carte blanche” to concrete over valuable habitats for wildlife.
The proposals would allow developers to compensate for the destruction of habitats by creating or enhancing other natural areas, but a cross-party report has found the system does not recognise the full value of the lost sites.
The Environmental Audit Committee said ministers should not press ahead with the scheme in England until pilot projects set up in 2011 had been properly assessed by independent experts.
The committee's Labour chairwoman Joan Walley said: "Biodiversity offsetting could improve the way our planning system accounts for the damage developments do to wildlife, if it is done well.
"But ministers must take great care to get offsetting right or they risk giving developers carte blanche to concrete over important habitats.
"Many witnesses to the inquiry were concerned that the Government's proposal would allow offsetting to be applied to ancient woodland and Sites of Special Scientific Interest. There is a danger that an overly simplistic offsetting system would not protect these long-established eco-systems."
She said the assessment process proposed by the Government was "little more than a 20-minute box-ticking exercise that is simply not adequate to assess a site's year-round biodiversity".
If a wood was examined in winter "it would be easy to overlook many of the migratory birds that may use it as habitat in the summer", she claimed.
The Government set out its proposals for an offsetting scheme in September, but the MPs warned against pressing ahead until the pilot projects had concluded.
The report said: "Arguably offsetting is an admission of failure in that it should only arise after alternative development sites or means of mitigating the environmental loss from development have been considered.
"A decision on the Government's offsetting proposals should not be made at this time. Offsetting pilots were set up in 2011 and these should be allowed to run their course and then be subjected to the independent evaluation previously promised by ministers.
"If that evaluation concludes that there are benefits in introducing an offsetting scheme, the Government should then bring forward revised proposals that reflect the concerns that we have raised in this report."
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