The EU is racing against time to stop genetically modified foodstuffs entering western Europe from the east after the community's enlargement on May 1, the Guardian has learned. Some of the 10 new member states have been growing GM crops for some time, but recent checks have shown that the testing facilities to monitor their spread to neighbouring crops are either flawed or non-existent.

The biggest agricultural country in eastern Europe, Poland, which has been growing GM crops for several years, has had no testing facilities at all.

Environmental groups accuse biotech companies such as Monsanto and Pioneer of using the former eastern bloc as a "trojan horse" to get GM products into the EU. However, these companies have been legitimately marketing their seed varieties there since 1996.

The problem is not lack of legal regulation. The EU has ensured that all the new members have rules on GM similar to those in the rest of the community. The difficulty is enforcement. Some of the newcomers have no idea whether their crops contain GM organisms since their testing regimes are inadequate. Where tests have been carried out by green groups some samples have been clear but others found to contain GMOs well above the EU legal limit for labelling.

The EU has recognised this as a problem and has been helping those countries without facilities to set up laboratories that can detect genetic modification in crops and foodstuffs.

Iza Kruszewska, a researcher for the Northern Alliance for Sustainability, an environment and development group, believes that by asking countries such as the Czech Republic and Poland to permit the commercialisation of GM maize before May "the biotech industry is trying to use the enlargement process to introduce GM by the back door of EU accession".

Beate Gminder, a spokeswoman for the health and consumer protection directorate of the European commission, disagrees. She says she is sure the problem of detection will be solved by May 1.

Each country will be responsible for certifying its own products.

"According to the law, all products containing GM will have to be labelled," said Ms Gminder. "If countries did not have the testing facilities or expertise to check their products they could contract the work to countries and laboratories that could do the work. I am sure everyone understands that."

She said the rules were clear. Some GM crops had been approved in the EU. If a food product contained more than 0.9% of an approved GM crop then it would have to be clearly labelled. Products containing more than 0.5% of crops - such as GM potatoes - that were not approved in the EU would have to be labelled as containing GM ingredients.

This second provision is an added hurdle for some of the 10 new member states because they have been growing crops not yet approved in the EU. Some of these may never be approved because they have been superseded by other varieties and have fallen out of fashion.

Geert Ritsema, the Friends of the Earth GM campaigner for Europe, said: "These regulations are all about the consumer's right to choose whether to eat GM or not. Poland has allowed growing of GM soya but without any regulations being implemented. People can buy and sell these things and plant contaminated seed with out fear of prosecution or detection because there is no method of doing so.

"After May 1 all edible oils will have to be labelled if they contain GM. Soya and maize oil would require a GM-free certificate. But in an unregulated country who knows whether the certificate means anything? If supermarkets want to be sure what they're selling to consumers they'll have to test the products themselves."

Besides the internal EU rules, he said the bio-safety protocol, which EU countries had ratified, made it illegal to export and import GM seeds without prior informed consent. Because of the history of growing GM in an unregulated fashion seed from eastern Europe needed to be tested to make sure it did not contain some contamination.

A second problem for Europe concerns some of the countries farther east, such as Ukraine, which have been growing GM potatoes since 1997, and candidate countries like Romania and Bulgaria, which wish to join the union in 2007. Romania, anxious to please the US, has grown GM crops on a large scale. Neighbouring Serbia accuses Romania of contaminating its supposedly GM-free crops as a result of grain smuggling across the border.

This is a particularly sensitive issue for countries such as Hungary, which has taken a strong GM-free stance to protect its seed-growing industry. EU states have been increasingly turning to Hungary as a source of GM-free seed. Remaining uncontaminated is a key to this continuing export trade. Hungary, along with the Czech Republic, is fully equipped with laboratories that can certify seed and food as GM-free. Most other new member states, while believing that their grain is GM-free, have no way of being sure.

Tony Combes, director of corporate affairs for Monsanto UK, rejected accusations of using eastern Europe as a trojan horse. He said: "Each accession country must comply with all aspects of EU rules and regulations to be full members - this includes the enforcement of product labelling in every industry. Equally, existing EU-approved GM crops may be marketed in accession countries once they have joined."

He added: "It is more a case of the EU being used as the standard to which the accession countries have to comply."