The UK government has approved the commercial planting in England of one variety of GM maize. BBC News Online looks at how the decision was made.
What exactly have ministers done?
The Environment Secretary, Margaret Beckett, has approved the commercial cultivation of one variety of genetically modified, herbicide-tolerant maize. This maize is intended to be fed to animals - not to humans.
It has been engineered so that it can be sprayed and still prosper while all the weeds around it die. Proponents of the crop argue this can improve yields and be kinder to the environment because the different field-management practices involved - the timing and frequency of spraying - can give wildlife opportunities to thrive that present practices deny.
So, this is not a blanket approval for all GM?
No. Margaret Beckett has said each GM crop must prove its case individually.
This means any biotech company seeking a licence for one of its products in the UK will have to put the crop through field trials (FSEs) similar to the ones which brought approval for GM maize. These three-year tests will establish, within their terms of reference, whether or not the crop is better than the conventional equivalent.
What advice has the government sought in making this decision?
The arguments over GM crops in the UK have raged for almost 10 years, but the serious business of making Tuesday's decision has taken place over the last three to four years.
In that time, the government instructed scientists to test GM crops, not just in the lab, but in the "real" environment of farm fields. An assessment of the economic benefits of GM has been undertaken. Senior government scientists have also reviewed the knowledge gleaned worldwide on the GM issue.
In addition, a public consultation programme was initiated. This process included a series of nationwide meetings which were open to all.
What did ministers learn from this?
The farm tests showed GM maize could be more beneficial to wildlife when compared with a conventional crop sprayed with the powerful chemical atrazine.
The economic review said there were few short-term benefits for the UK in pursuing the technology but there could be longer-term benefits for farmers and consumers from crops that were better suited to the British climate and which had boosted nutrients.
The science review said the risks to human health from GM were "very low" but there were concerns about the impact the crops could have on the environment.
The public consultation produced perhaps the clearest advice for the government. More than half of Britons who took part in a nationwide debate on genetically modified crops said they should never be introduced under any circumstances.
What happens next?
The GM maize variety concerned is called Chardon LL, and it is a spring-grown crop. The earliest possible date the maize could be planted is in early 2005. First, it must be placed on the UK Seed List (the national list of varieties).
The government will also need to seek the approval of the Pesticides Safety Directorate for the chemical - glufosinate ammonium, marketed as Liberty - to be sprayed on the GM maize. Other key issues also need to be resolved. The Welsh and Scottish assemblies technically have a veto of the licensing of GM - even if the crops are to be grown only in England.
Concerned groups will also want assurances on co-existence (whether GM crops can be planted close to conventional or organic ones without contaminating them) and liability - who pays if something goes wrong.
Which crops are next?
Biotech companies have a range of trait-specific crops they would like to introduce. These plants include varieties that are toxic to insect pests and so should not suffer so much wastage or need as much chemical treatment. All these novel plants are likely to have to follow the FSE path if they want to be commercialised in the UK.
Remind me, exactly what are GM crops?
They are plants that have had genes added in the laboratory to make them perform in a very specific way. This modification can, for example, produce crops that grow more favourably on poorer soils or have human proteins in their tissues that can be processed to make medicines.
Conventional breeding can shuffle genes between closely related plants but it cannot move genes between very distant relatives or incorporate genes from animals and bacteria. Only GM can do this.
This has led to maize that kills its main insect pest if it tries to eat the plant, and tobacco whose leaves can be used to treat stunted growth in children.