Justice Secretary Chris Grayling has admitted that prison staff recorded and listened to confidential telephone calls between more than 30 MPs and prisoners.
The minister apologised and confirmed there would be an urgent investigation into the tapping, which took place between 2006-2012.
In an emergency statement to the House of Commons earlier this week, Mr Grayling disclosed that several private calls between inmates and their lawyers had also been monitored.
A call between a prisoner and the office of Lib Dem MP Simon Hughes, who is now Mr Grayling’s deputy at the Ministry of Justice, was among the telephone conversations intercepted.
Reacting to the news, Mr Hughes said: “I am angry. I am also absolutely clear we need to take action to prevent it happening, which is what has been done as quickly as humanly possible by the department responsible.
“I am very concerned that we continue to uphold civil liberties.”
Mr Grayling insisted there was no evidence that information from the phone calls had been passed on to anyone else and claimed the conversations had been monitored because of mistakes in the routine checking process.
Nonetheless, the news has enraged human rights activists and members of the opposition.
Shadow Justice Secretary Sadiq Khan said the matter was “a very serious breach of confidentiality” and there were many questions still to be answered.
“There are very good reasons why correspondence between MPs and their constituents is confidential, wherever they live and whoever they are. Any breach of the long-held rules must be treated very seriously, which is why we need to get to the bottom of this urgently,” he said.