Interpreters from a private company have been working in courts in England and Wales without the required criminal record checks, the BBC has been told.
The checks are "non-existent", one interpreter said.
A private company, ALS, took over the £300m contract earlier this year but has faced criticism from judges, politicians and lawyers.
ALS says if it finds interpreters who do not have the right documentation, it will remove them from the register.
"Edward" (not his real name) has been a court interpreter for eight years and has worked for ALS since March.
He does not want to be identified for fear of losing work but describes security checks as "non-existent".
He says he registered with ALS one evening and was offered his first work the next morning.
"I just did it online. I hadn't filled in my references or work experience; they rang me up the next morning and offered me jobs. There was no chance to check anything; they didn't," he told the BBC.
It was "impossible" to check if he had a valid Criminal Records Bureau (CRB) check, he said.
At that time he did not have a valid CRB check as required by ALS and the Ministry of Justice.
He worked without a CRB check for three months. In that time he worked on one trial involving Chinese triad gangs.
Edward fears lax security could allow criminals access to sensitive court information. It would, he says, be "pretty easy" to falsely register as an interpreter.
He describes that as "very worrying".
That fear is echoed by Neil McCafferty, who is projects director of translation company Talk Russian UK.
His wife had been an occasional court interpreter and last year, to test the new system, he registered his cat Masha with ALS.
He said: "We signed her up for the rare cat language. We were absolutely staggered to start receiving emails from the company suggesting we take Masha the cat for a language assessment".
In July this year he registered again but in his own name. He has no valid qualifications and no CRB check.
He said: "I was absolutely shocked when on their mobile phone app it was offering me work - I could have been anybody."
"I could have been trying to access the criminal justice system for all sorts of unpleasant reasons. That in itself is incredibly frightening," said Mr McCafferty.
There are 800 requests a day from courts in England and Wales, for an interpreter to assist a witness, a victim, or a defendant.
ALS, now owned by outsourcing giant Capita, has also been criticised for not providing enough staff and for low standards.
The most recent figures show that between 30 January and 30 April there were 2,232 official complaints about the service.
Of the trials it was asked to attend, it managed to provide staff to 81% - its contract demands 98%.
Translation errors have led to trials collapsing.
A Ministry of Justice spokesman said: "Interpreters working in HM Courts and Tribunals must be vetted to the minimum requirement of an enhanced Criminal Record Bureau check".
"It is the contractor's responsibility to make sure they meet this requirement. We keep this and other contractual requirements under scrutiny," he added.
It confirmed 43 interpreters had already been removed from the register for not having the appropriate checks.
ALS said: "If the BBC is aware of any interpreters working without the necessary information and is prepared to provide specifics, ALS will investigate and suspend or remove such interpreters, if appropriate."
ALS was started from his back bedroom by Gavin Wheeldon in 2003. Eight years later he won the contract with the Ministry of Justice.
In December 2011 he sold the business to Capita and until last month was chief executive.
He strongly defends the performance of the contract: "Month one was very very bad, month two it improved dramatically...the level of complaints are almost non existent now."
But "Edward" is turning his back on a system he thinks is insecure.
"I'm not an isolated case, I talk with other interpreters, many of them have no qualifications at all," he added.