By Alex Kirby
BBC News Online environment correspondent
Major new field tests should be done before any genetically modified crops are allowed to be grown commercially in Britain, MPs have told the government.
Results from GM and conventional crop trials have been mixed
The Environmental Audit Committee report comes days before ministers are expected to approve the commercial planting of a type of GM maize.
The UK's top scientists, the Royal Society, say there is no doubting the validity of the original test results.
But the committee calls approval on the basis of the tests "irresponsible".
It says the trials on the maize were invalid, because the ordinary maize used as a comparison in tests with its GM equivalent was sprayed with a powerful weed killer, atrazine, which is to be banned across the European Union.
The unanimous all-party committee report calls the trials "an unsatisfactory, indeed invalid comparison", and says new trials should last at least four years.
The crop is believed less damaging to wildlife than its conventional version.
The trials sought to assess the GM crops' impact on wildlife, but the committee says: "We are very concerned about possible contamination by gene-flow and pollen spread to non-GM crops, and insist that the issue of liability be settled before any GM crops are allowed to be commercially grown in the UK."
Scientists say that even after atrazine is withdrawn, the GM maize will still prove better for wildlife.
The scientists, led by Professor Joe Perry from Rothamsted Research and writing in the journal Nature, acknowledge the maize will not perform as well as it did in the trials.
But they believe it will still let more weeds flourish than conventional maize, helping birds and insects to survive.
They are from the consortium which carried out the four-year trials on GM crops, known as the farm-scale evaluations.
In the trials, fields sown with genetically modified maize produced more weeds and seeds than those planted with conventional maize.
With two other trial crops, oilseed rape and beet, the conventional fields emerged more wildlife-friendly than the GM ones.
The independent body, the Advisory Committee on Releases to the Environment (Acre), told the government growing GM maize would not harm wildlife.
But both Acre and the scientists acknowledged the concerns of GM opponents, who said the maize trials had been fatally flawed by the use of the virulently powerful atrazine.
Their argument is that the supposedly benign GM maize might lose its advantage against the conventional crop if that was treated with something less devastating than atrazine.
The scientists say withdrawing atrazine will lessen the comparative benefits for wildlife of GM maize, but will not cancel them altogether.
Minutes of a Cabinet committee meeting seen by BBC Two's Newsnight suggest qualified approval for sowing GM maize is about to be given by the government.
Peter Melchett, Soil Association policy director, told BBC News Online: "We applaud the Environmental Audit Committee for saying what we've been saying for the last five years - the biodiversity benchmark for GMs should be wildlife-friendly crops like organic ones.
"It's a key recommendation to ask whether GMs can be as good as or better than the best alternative, not the worst.
"But the government does now seem prepared to ignore not only the public and its own advisers but also Parliament in forcing GM crops on us.
"The committee is right to say ministers should not go ahead 'on the basis of one narrow component of the entire evaluation of GM technology'."
But a spokesperson for the Royal Society, the UK's national academy of sciences, said: "The scientific robustness and validity of these results are not in doubt and were thoroughly assessed by independent peer reviewers before publication.
"The committee recommends against granting permission for the growth of GM maize under the conditions tested in the farm scale evaluations. However, this flies in the face of the independent scientific advice from Acre."