A judge expressed alarm yesterday at the standard of interpreters used by Norfolk police after being told an interview was held in Russian by a woman who had studied the language only at school.

Judge Peter Jacobs spoke out after claims by a defendant in a trial of three foreign nationals that a Latvian interpreter conducted a police interview in Russian, despite the fact that she was not qualified to translate the language.

Norwich Crown Court heard that she had failed an exam to act as a Russian interpreter, but because she had been taught the language at school ?she had a stab at it?.

Another defendant in the case has also made complaints about the fact that his police interview was conducted in Russian by a Latvian interpreter and that this was not brought to the attention of the custody sergeant.

Judge Jacobs described the situation as ?unsatisfactory? and demanded to know why it had arisen.

Last year, solicitors in Lincolnshire complained about the quality of interpreters, prompting a report featured on the BBC's Politics Show, and last week a Norfolk-based interpreter raised her concerns with Home Secretary Charles Clarke.

The National Union of Professional Interpreters and Translators said last night that in outsourcing its interpreting services from Cambridge-based agency Cintra, Norfolk police were using translators who were not all on the official register stipulated in government guidelines.

And it warned that where unqualified interpreters were used or interpreters were not qualified in the language being used in a police interview, cases could end up being thrown out of court.

While every interpreter used by Suffolk and Cambridgeshire police is on the National Register of Public Service Interpreters (NRPSI) and those forces could provide figures for the exact number of cases in which they were employed, Norfolk police could not be sure whether those they used were on the register, said the union.

Only 31pc of interpreters supplied to Lincolnshire police since they signed a contract with Cintra were on the national register, and there are no figures for Norfolk police, according to figures it has obtained.

Prof Guillermo Makin, the union's campaigns officer, said that because of the pay offered by agencies, many interpreters who featured on the national register did not want to work for them.

But under best practice recommendations laid down by Lord Justice Auld, ?every interpreter working in the courts and police stations should be registered with the National Register of Public Service Interpreters?.

In contrast to forces which employed interpreters directly from the NRPSI, Norfolk and five others were in danger of using unqualified translators.

?The national register does not have a call centre like solicitors have,? said Prof Makin.

?Custody staff can be busy, and so if they need a Slovakian interpreter that can be difficult. That is the only advantage of outsourcing it to an agency.

?But if they don't receive a properly qualified interpreter in the right language or an interview is done in a second language, they are in real trouble.?

Christa McGrath, chief executive of Cintra, said the agency had access to 800 interpreters on the national register and 350 more who were qualified but chose not to pay the ?200 fee to feature on it.

All translators provided by Cintra were fully qualified, she added.

Claire Bailey, diversity projects officer for Norfolk police, said there was a real shortage of interpreters in several languages, particularly those from eastern European countries.

Records for January showed Norfolk police made 100 calls to Cintra for face-to-face interpreters.

As they were provided by Cintra, the force did not have details about how many of them were on the national register.