A team of UK Government advisers has given no clear direction to ministers on whether to commercialise GM crops.
GM maize is the closest to being commercialised
A report from the Advisory Committee on Releases to the Environment said a new modified maize should only be grown if its cultivation mirrored recent trials.
It added that weedkiller-tolerant beet and rape should not be commercialised if their planting followed the tests, as this would probably harm wildlife.
Ministers will now consider the advice before reaching a final decision.
Acre's deputy chairman, Professor Jules Pretty, said: "This is neither a green light nor a death knell for GM.
"We're saying 'yes, but' to the maize and 'no, but' to rape and beet." He added: "It's not 'yes, no, no' - the buts are very important."
Acre was asked by the Environment Secretary Margaret Beckett to assess the outcome of three-year farm-scale trials of the maize, beet and rape - all of which had been engineered to thrive in the presence of specific herbicides that would damp down competitive weeds.
Acre confirmed the studies' findings, that the trials on GM maize did not demonstrate evidence of adverse environmental impact, while GM sugar beet and rapeseed testing indicated the environment could be at risk - with fewer insects and seeds to support farmland birds, for example.
"Based on the evidence provided by the farm-scale evaluation (FSE) results, if GM herbicide-tolerant (GMHT) maize were to be grown and managed as in the FSEs this would not result in adverse effects," its report said.
"At the moment we are saying that oilseed rape and beet shouldn't be given consent but we can imagine ways of farming them that could lead to positive effects on biodiversity on the landscape," Professor Pretty told the BBC.
The trials are a key part of the government's decision-making process over whether UK farmers should be allowed to grow engineered crops commercially.
The government says the GM maize has received qualified backing from English Nature, its independent wildlife watchdog.
The Department for Environment, Food and Rural Affairs quotes English Nature as saying the crop might be commercialised in Britain, on certain conditions.
None of this means GM maize will necessarily be grown in the UK in the near future, though in principle the first plantings could take place this spring.
The control crop grown alongside the GM maize was managed using the herbicide atrazine, which is being phased out by the European Union - potentially making the results of the maize FSE irrelevant.
Professor Chris Pollock, the chairman of Acre, said: "From the time of possible approval of GM maize to the time when the approval of atrazine expires would be about 18 months.
"After that you would have to show that the lack of impact on biodiversity achieved by the GM crops in the FSEs still compared well with the new method of weed control for conventional maize."
Mrs Beckett welcomed the report from Acre, adding: "We will now consider Acre's advice, as well as the advice from English Nature, very carefully before reaching a view on whether these crops should be approved for cultivation in the EU.
"I have said consistently that the government is neither pro- nor anti-GM crops - our over-riding concern is to protect human health and the environment, and to ensure genuine consumer choice."
Reacting to the Acre report, Dr Mark Avery, from the Royal Society for the Protection of Birds (RSPB), said: "Farmland bird populations have been in freefall for more than three decades.
"The cultivation of GM beet and oilseed rape would drastically reduce seed numbers and put familiar birds such as the skylark and yellowhammer in even greater peril.
"This is a 2-0 result against GM crops. The government should not allow GM herbicide tolerant beet or spring oilseed rape to be grown commercially in the UK.
He said that the impending removal from use of atrazine undermined the case for GM maize.
Dr Paul Rylott, of the Agricultural Biotechnology Council (ABC), countered: "These recommendations confirm what the industry has always believed, that grown in the right way, GM crops can benefit the environment."
He added: "Activist groups claimed that GM crops were in effect 'green concrete' and would 'wipe out' wildlife. But Acre considers that it is 'possible to manage weeds using GMHT beet (or oilseed rape) such that the impact on biodiversity is less or comparable to that of conventionally managed beet (or oil seed rape)'.
"ABC is and always has been, fully committed to the stewardship of GM products, such that they are grown in a responsible way that benefits consumers, farmers and the countryside".