Allegations over the bugging of Tooting MP Sadiq Khan and a constituent he was visiting in jail have sparked a furious row over the police's actions and who authorised them.
There are concerns that telephone conversations in prison are bugged
Following the news of this surveillance activity, Simon Creighton, of Bhatt Murphy solicitors, has revealed that transcripts of two telephone conversations he had with a client were inadvertently sent to him by a government lawyer.
This indicated that his own dialogue with prisoner Harry Roberts, jailed for killing three police officers in 1966, had been recorded.
Mr Creighton now has highlighted the fears among some defence solicitors and their clients that their conversations, conducted in prison, are being monitored.
"All telephone calls made by prisoners are capable of being monitored," Mr Creighton said.
"But the law requires that calls to lawyers are legally privileged and all monitoring should cease.
"But it's being made apparent that calls between myself and one of my clients were, in fact, recorded and transcripts of those calls have been supplied to me."
Mr Creighton said that he had made this discovery "very recently", when the transcripts of conversations with Mr Roberts at Channings Wood Prison, Devon, were given to him by a "rather roundabout route".
He said: "The calls that were monitored... took place quite some time ago.
"There has always been a suspicion amongst people in prison that their privileged calls are monitored.
"One of the problems that people in prison have is that all their communications with the outside world are subject to monitoring by the authorities.
"And so there is a feeling that you have to depend on the good grace of the authorities in order to respect the legal right to confidentiality.
"Now, prisoners have always suspected that this isn't respected, and we now seem to have proof that this may be happening on a more routine level than anyone previously suspected."
Mr Creighton said that the principle of being able to seek confidential legal advice was a "very, very basic principle of English law".
He said: "It's a principle that's been upheld by the highest courts in the country on many occasions, as well as the European Court of Human Rights.
"So it's rather worrying if the rule of law is being ignored in this way."
Mr Creighton said there were also implications for trials and, in particular, those which involve the government.
"The real worry is that, very often, the case is being brought against the government, and if the government have the knowledge of exactly what's happening in your case and how it's being prepared, then it gives them an unfair advantage from day one."
Mr Creighton also said that the case involving Mr Khan would only further existing fears that bugging in prisons is already taking place.
"It's certainly the case that many prisoners have long been worried that their supposedly legally privileged, confidential calls have been monitored.
"And obviously, discovery of this has happened on at least one occasion [and this] will give rise to further paranoia on the part of people and further suspicions that it does indeed happen more regularly.
Privileged communications were reserved for people who could be trusted to use them responsibly, he continued, "so they are restricted to the legal profession, to MPs.
"If you take the view that people shouldn't be afforded the right to legal privilege, then you should take that away from them, you shouldn't undermine it by the back door."
But a spokesman for the Prison Officers Association denied that conversations between prisoners and their legal representatives were monitored.
"Prisoners' general phone calls are recorded and a small percentage are listened to randomly, to ensure that they are not organising a prison break or for drugs to be brought in," he said.
"However, if a prisoner calls their solicitor then that call is not recorded at all. If prisoners felt that the phone calls they made to discuss their legal cases were being recorded then there would have been a general outcry before now.
"Suggesting they are recorded is a general assumption that's wrong."