Government guidance on recruiting supply teachers from abroad concedes that, in some countries, potential recruits will be unable to obtain certificates stating that they have no criminal record.
Schools and local authorities are instead advised to be "especially vigilant" in carrying out reference and other background checks.
The countries concerned include Russia, Pakistan and Mexico. British teachers must undergo police checks before working in schools here.
The Government's admission comes as many schools are scouring the globe for teachers to fill vacancies in September. Eamonn O'Kane, general secretary designate of the National Association of Schoolmasters Union of Women Teachers, said he would be concerned if standards were less rigorous than for British teachers.
He said: "If the police checks are not there in certain countries, there is no way employers can make those checks from over here. But I think it's dangerous. If there was a problem in a school, I'm not sure parents would be particularly happy with the explanation that the authorities were not able to carry out police checks overseas."
However, one recruitment agency, which is training and placing eight teachers from St Petersburg in London schools, said the recruits could not gain a work permit to leave Russia if they had a police record.
Roger Cole, director of the firm International Professional Relationships, said that all the teachers, though lacking a police certificate, had been subject to detailed background checks by the British consulate in St Petersburg before even being allowed to attend interviews in Britain.
One of the largest supply agencies, TimePlan, has collated information on the checking mechanisms of 55 countries. Australia, New Zealand, South Africa, Canada and the United States - British schools' main recruiting areas - issue certificates showing a clean criminal record.
Eight other countries - Bangladesh, Guyana, Hong Kong, Malawi, Malaysia, Mexico, Pakistan and Russia - do not, says the agency. TimePlan says it does not recruit from countries which cannot provide police certificates.
Another agency, Eastbourne-based Morton Consultancy, which recruits teachers from Bulgaria, said it would not take Russian recruits because of the absence of the police certificate.
A Department for Education and Skills spokeswoman said the "vast majority" of supply teachers were from countries which did provide police certificates. She added: "There are lots of other checks that schools and local authorities can carry out. For example, we recommend that employers don't just rely on references, but insist on original documentation, communicating directly with referees and cross-checking references."
Recruitment expert John Howson, visiting professor at Oxford Brookes University, is calling for the Government to provide compulsory, four-week training for all overseas recruits entering classrooms in September. He said ministers should also investigate giving all those in their first year in Britain guaranteed non-contact development time. The call comes after The TES reported last week that some overseas staff have been offered virtually no training before starting work in schools.