The attack, at the Jealott's Hill site in Bracknell, Berkshire, belonging to Zeneca Plant Sciences, was on Sunday night. In the process, protesters destroyed an equal number of normal poplars used as comparisons in the trial, which began in 1995 and had the approval of English Nature. Five trees survived.
The trees had been engineered to contain less lignin, the "woody" element of trees, so that less chlorine would be needed in the bleaching process during paper-makingand less energy needed to turn the trees into paper.
Anonymous activists claimed responsibility for the damage, in which younger trees were broken, exposing pink wood that indicated their genetic modification. Older trees - some up to 30 feet tall - had a ring of bark stripped so that they will slowly die. Dr Nigel Poole of Zeneca called the process "like cutting someone's artery and letting them bleed to death".
Karen Holt, the project manager, said: "I planted the more mature trees myself so I was very sad to see them destroyed. The environment will suffer as the research was demonstrating that less energy and chlorine would be used in treating wood pulp for paper. An eight-year research programme has been seriously affected, and trees damaged have been condemned to a slow death."
Biotechnology companies are legally obliged to publicise the sites of trials, but cannot police them around the clock. Scores of sites have been damaged or crops ripped up in the past two years as the GM products furore has heightened.
The Zeneca trial was approved in December 1996 byEnglish Nature, which said there was "negligible risk to our native flora and fauna" and that "the fact that flowering will not take place will prevent any possible outcrossing". The trees will not flower because they are all female, so will not produce pollen.
Fears that GM pollen from herbicide-resistant crops would interbreed with weeds have often been cited in opposition to GM crop trials. However, trees with less lignin would generally be weaker than their normal cousins, so that trees with the "new" genes would have a lower chance of survival in the wild.
In a statement, the activists said that genetic modification of trees "is a major threat to the world's environment" and that "GM trees will lead to increased conversion of forests into regimented wood pulp factories, and increased chemical use". They added that the use of female trees "ignores the issue of horizontal gene transfer" caused by viruses or bacteria.
Zeneca had been worried that the site would be a target. In April, Dr Poole said: "I imagine they're going to try to cut the bark and destroy the trees. It's not about food, it's not about the environment. It's just sheer vandalism."