Frequent failings in court translation services are damaging faith in the legal system and could lead to miscarriages of justice, lawyers and magistrates have warned.
Some translators have been advising witnesses how to answer questions, coming up with questions of their own or sometimes simply failing to translate what is happening.
It is said the problem is getting worse as the quality of translation services declines.
The Bar Council, which represents 18,000 barristers in England and Wales, claims some interpreters fail to understand that their role is simply to translate what is being said.
Instead they have conversations with witnesses which can include "summarising, commenting, asking their own questions and advising".
In one case a witness was provided with an interpreter "who told her what to say and what not to answer", according to a barrister in the courtroom.
The Bar Council warned in evidence to a House of Lords inquiry: "Those examples are discovered frequently but, given the fact that normally no one else in court speaks the language, there will undoubtedly be many more that occur that are not caught."
The Magistrates' Association highlighted a trial in which the defendant had to act as a translator because nobody else was available.
A magistrate said: "We ended up with a Romanian interpreter when what we needed was Roma - for the mother of a youth defendant. The defendant ended up interpreting himself and nobody knew whether what he was telling her was anything like it should have been."
The Magistrates' Association said: "The current failings in interpretation and translation services could lead to miscarriages of justice."
In evidence to the same inquiry, the Law Society, which represents 200,000 solicitors, said: "The lack of availability of interpreters and reliable translation services has an impact on the running of the courts and on public trust in the justice system."
Barrister Philip Stott, co-chair of the Legal Services Committee at the Bar Council, told the inquiry: "My sense is that the quality of interpretation has gone downhill right across the board."
The Ministry of Justice has an official complaints process for people involved in court proceedings to report poor interpretation services.
Its spokesperson said: "Over 99 per cent of all court translation services are completed without a hitch - ensuring victims, witnesses and defendants get a fair hearing and trial.
"Interpreters are only permitted on the register if they can demonstrate competence.
"In the small number of cases where standards have not been as expected, we encourage people to make use of the official complaints process so we can improve services."
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