Jay's brief is to deliver answers for Leveson, who will be at pains to be as impartial as possible with News Corp boss and son 

 
He has prosecuted multinationals for dumping toxic waste off the west coast of Africa and defended the government over alleged colonial atrocities in Kenya. And on Wednesday Robert Jay QC will come face to face with Rupert Murdoch at the Leveson inquiry.

With his yellow-framed glasses, tight-cropped beard, and mild-mannered style of questioning, Jay may seem unsuited for the task. Over the last five months acting as lead counsel to the inquiry, he has been thorough and forensic in his questioning of dozens of witnesses, but rarely has he been pugnacious. Even lawyers who know Jay and have instructed him in the past say he is far from an attack dog.

A leading silk in administrative and public law, Jay's most high-profile case up to now was against the oil-trading company Trafigura in 2007, which agreed to pay £30m in compensation to 31,000 people in Ivory Coast who claimed they had fallen ill after toxic waste was dumped on rubbish tips , poured down drainsand left at roadsides.

"His strength is he is very good at getting on top of his brief, in terms of getting to the detail. Without any doubt, he will know his subject inside out. He gets to the office at 7.30am or 8am, he puts the hours in to get the detail. Judges love him," said Martyn Day, partner with Leigh Day solicitors who instructed him on the Trafigura case. Sources say Jay has "capped" his workload during the Leveson inquiry at 70 hours a week.

Day agrees that Jay is no rottweiler but argues this should not be seen as a weak link in the Leveson inquiry's armour. It is not a criminal trial, he points out, and Jay's brief is to deliver the answers for Leveson, who will be at pains to be as impartial as possible with both Rupert and James Murdoch, who appears on Tuesday.

Day added of Jay: "He won't go for the jugular. He is not somebody who is going to go after Murdoch in that sense. He will go for him in an intellectual way but not in an emotional way, which is exactly what Leveson will want. Leveson will not want to be seen to be partisan."

The 52-year-old Jay is the leading counsel in a team of three barristers who have grilled witnesses since November. One lawyer linked to the inquiry said: "He seems like the antithesis of a barrister because you are used to barristers going in hard trying to score points.

"Robert is doing exactly what Leveson wants him to do, which is not to be partisan. Occasionally he goes up a gear but most of the time he stays in inquiry mode with questions purely designed to get a factual answer without probing."

And this, say those close to Jay, is what the barrister is relishing: a historic opportunity to quiz Murdoch about the facts, not opinions and hearsay about his relationship with politicians, particularly David Cameron, Tony Blair and Margaret Thatcher. "He relishes complex litigation involving intricate factual disputes and contested expert evidence," said one source.

Jay is said to have been voraciously reading up about Murdoch. It is this appetite for detail and his intellectual ability that led him to being selected as Leveson's lead counsel, catapulting him into a showcase inquiry and a hefty salary. Of the £1,992,600 total spent on the inquiry up to 31 January, £536,100 has been used to meet the costs of a team comprised originally of three barristers, of whom Jay will take the largest share.

Jay graduated from New College Oxford in 1980 with a first-class honours degree in jurisprudence, became a junior counsel in 1989 and a deputy high court judge in 2008.

Apart from Trafigura, he has made his name on several high-profile cases. He acted for the government in a case about alleged crimes in the Mau Mau uprising in Kenya and acted for Defra in the pesticides action brought by campaigner Georgina Downs (won by Defra in the court of appeal).

Outside work he eschews the networking and excesses many would associate with high-flying barristers. He is said to be interested in politics but "absolutely apolitical" and has a passion for history, art and classical music, opera in particular. "His greatest love is Wagner and the Ring Cycle," says one friend. He loves to cook for his author wife, Deborah, and their daughter. He cycles everywhere, plays golf and has little interest in fashion. "He once turned up at a meeting with shorts and striped socks and shoes," said one lawyer.

He has grilled about two-thirds of the witnesses so far and some fear he may have taken on too great a workload. That is the generous explanation for giving the editor of the Sun, Dominic Mohan, such a light grilling when he appeared. Others say Jay was deeply unimpressed with Mohan's coached performance and was saving up for the day he would meet his master.

Another high-profile witness quizzed by Jay is the former Mirror editor Piers Morgan. At times the exchanges bordered on the bizarre. Morgan was asked about a story the paper had decided against publishing about a switchboard operator who had psychiatric problems but had observed they were the same problems a paedophile might have.

"What is the difference?," asked Jay. "Between a paedophile and somebody who runs a switchboard?" Morgan replied. "Yes," said Jay. "I would have thought it's fairly self-evident," retorted Morgan. "Yes, but explain it to us, please, in this context," said Jay, lining up Morgan for a facetious reply. "Well, one is, you know, potentially abusing and raping young children, and the other one is manning a switchboard."

But Jay also has a razor-sharp ability to cut a witness down to size. When Richard Desmond, owner of the Express and Star newspapers, tried to argue that Madeleine McCann's disappearance was just like Princess Diana's death and this was why his paper had libellously speculated that the child's parents had a hand in her murder, Jay interrupted: "Mr Desmond, I'm beginning to sound irritated, but I am. There is no comparison between these two cases because, to be absolutely stark about it, in the case of Princess Diana we have a dead body. What has that got to do with the McCann case, please?"

Like all witnesses, the Murdochs have been offered a meeting with Jay before they appear to enable them to "understand the purpose and direction of the questioning".

It is yet to be seen which version of Rupert Murdoch Jay will face. "There are two public Murdochs," said his biographer, Michael Wolff, author of The Man Who Owns the News. "There is a prepared Murdoch in which all corporate resources are brought to bear, PR resources, legal resources. When he is in preparation mode, he rehearses a lot. In that case he will do the right thing. He will be precise and as sparing as possible in his answers. He will be on script."

The other Rupert Murdoch, said Wolff, "is an incredibly impatient man who swats people away, who becomes a rogue player, especially if he is tired, hasn't had enough sleep". In that case, "it's shoot-from-the-hip Murdoch, he will ramble, he will lose the point, he will get angry".

Murdoch's appearance before Lord Justice Leveson's inquiry will focus on the network of personal and professional ties that have bound his newspaper and television operations to some of the most senior politicians, and this is where his testimony could even be dangerous for the prime minister.

The Radio 4 Media Show presenter and Leveson watcher Steve Hewlett said that if Murdoch decided to "dish the dirt" on certain political meetings, "then Jay will have delivered. He will have set Leveson up nicely for the next module when Cameron and Blair are in. And that is what this is all about. Then we will have the mother of all battles on our hands."