Tom Crone, who left News International in July 2011, was cross-examined on Tuesday before a five-person disciplinary tribunal for the Bar Standards Board, sitting in the heart of legal London at Gray's Inn.

He faces three charges of professional misconduct, for engaging in conduct being discreditable to a barrister, and in conduct likely to bring the profession into disrepute.

He faces being disbarred as a barrister if the tribunal finds against him.

In a series of testy exchanges with BSB barrister Christopher Aylwin, Mr Crone denied he intended to blackmail his legal opponents into quitting the phone hacking actions.

He insisted it had been a legitimate attempt to gather "circumstantial evidence" that the solicitors had been improperly sharing confidential information damaging to News of the World with each other and with The Guardian newspaper.

By that time News of the World was dealing with about ten claims for compensation over phone hacking, from victims including Sienna Miller.

"It was your intention to use evidence of the affair to persuade, and I use the word neutrally, to persuade [lawyers] Ms Harris and Mr Lewis to remove themselves from the litigation, that was the point of it," Mr Aylwin said.

"No it wasn't," Mr Crone replied.

Mr Aylwin continued: "You would not have issued a barefaced threat ... but there are ways and ways of getting your point across."

"You would have used that evidence and consequent fear of embarrassment that would accompany a threat of disclosure to apply pressure on those two solicitors".

However Mr Crone said "evidence and information, confidential information disclosed within the cases, usually by us, was popping up in The Guardian" and though he did not have any direct evidence about the source of the information being leaked, "if we could prove they were having an affair, it clearly adds circumstantial weight to that allegation".

Such evidence could potentially be used in a professional standards investigation against the lawyers that could result in their being replaced in the phone hacking actions.

Mr Crone said approaching the lawyers with evidence of an affair would "qualify absolutely as blackmail" and he would never have done so, he would only have passed it on to News International's own solicitors and sought further legal advice on the proper way of proceeding..

Anyway, the surveillance did not find any evidence of an affair – indeed the investigator appears to have followed the wrong person at one point.

"It was a complete shambles," Mr Crone said, saying he personally regretted both the operation and its consequences.

Former News of the World news editor Ian Edmondson told the Leveson Inquiry he used private investigator Derek Webb "to carry out investigative work on quite a few occasions".

He said he did "on the express instructions of Tom Crone ask Mr Webb to investigate Ms Harris and Mr Lewis."

"Tom Crone told me that he had a great story that he wanted me to investigate and went on to talk to me about Charlotte Harris and Mark Lewis.

"He told me they were lawyers involved in phone hacking cases and that he suspected they they were having an affair and that confidential information was being passed back and forth between them."

Mr Edmondson said his response was "to express very considerable surprise and to say that I could not begin to see why the newspaper would want to run such a story. Tom Crone's response was that he accepted that ... but told me that the main reason to investigate was that it could provide the newspaper with good leverage against the two individuals."

Mr Crone denied saying the information would provide good leverage. "It was all to do with leveraging them off the cases."

He said he had asked for a "sensible reporter" to be put on the investigation, however he denied he had anticipated it would be used for a story in the paper.

Former Formula One boss Max Mosley – who was infamously the subject of a News of the World "sexposé", then sued the paper and won – was at the Inn to watch his former legal opponent cross-examined.

If the panel finds Mr Crone guilty they have the power to issue a warning, a fine, to suspend him from practice or to disbar or disqualify him altogether.

Mr Crone was the lawyer for News of the World's High Court defeat by Max Mosley, and its victory over Jeffrey Archer.

The Independent once wrote that he "knows where many of (News Int HQ) Wapping's bodies are buried".

He was arrested in August 2012 as part of the police investigation into phone hacking but in October 2014 the Crown Prosecution Service announced there was "insufficient evidence for a realistic prospect of conviction".

The tribunal is due to hear final arguments and reach a verdict on August 15.