James Murdoch insisted on Friday night that he had answered questions "truthfully" to MPs earlier this week after two former News International executives accused him of misleading parliament.

As David Cameron suggested that News Corp shareholders may like to review the management of the company, Murdoch wrote to the Commons culture, media and sport committee to say he stood by his evidence. He wrote to John Whittingdale, the committee's chairman, after the campaigning Labour MP Tom Watson referred him to the police. Watson acted after Colin Myler, former editor of the News of the World, and Tom Crone, former News International legal manager, issued a joint statement on Thursday night saying that Murdoch had been "mistaken" when he said he had not seen a key email. Whittingdale said he will now seek further clarification from Myler and Crone.

Known as the "for Neville" email, this challenged the company's core defence that only one "rogue reporter" had been involved in phone hacking. The email raised the possibility that Neville Thurlbeck, the paper's former chief reporter, was also involved. Watson, a member of the committee, asked James Murdoch about the email when he appeared before MPs on Tuesday.

The email is understood to have been decisive in the decision by News International to make an out-of-court settlement to Gordon Taylor, chief executive of the Professional Footballers' Association. James Murdoch said earlier this month that he was not given a "complete picture" when he approved the payment.

Watson told the BBC: "It shows that he [Murdoch] failed not only to report a crime to the police but because there was a confidentiality clause in the settlement it means that he bought the silence of Gordon Taylor. That could mean he is facing investigation for perverting the course of justice."

In his letter to Whittingdale, Murdoch wrote: "Allegations have been made as to the veracity of my testimony to your committee ... As you know, I was questioned thoroughly and I answered truthfully. I stand by my testimony. I am preparing a written response to the questions that I undertook to follow up on when I appeared before you on 19 July."

The prime minister said: "Clearly James Murdoch has got questions to answer in parliament and I am sure that he will do that."

Cameron also indicated that shareholders may want to review the News International management. "Clearly News International has got some big issues to deal with and a mess to clear up," he said. "That has to be done by the management of that company. In the end the management of a company must be an issue for the shareholders of that company."

The renewed pressure on Murdoch came amid growing nerves in Downing Street about the impact of the hacking scandal on the prime minister. Aides believe Cameron has not put a foot wrong since cutting short his visit to Africa and returning to Britain late on Tuesday night ahead of a marathon appearance in the Commons the following day.

Tory MPs showed their appreciation for Cameron's appearance, in which he admitted it was a mistake to hire the former News of the World editor Andy Coulson, by banging desks when he appeared before the 1922 committee later that night. The prime minister was said to have put in a confident performance.

Officially No 10 is saying that voters are not interested in phone hacking. Behind the scenes, however, there are nerves that No 10 has no control over the legal process and questions about Cameron's judgment in hiring Coulson which will not go away.

One senior Tory said: "A coat of Teflon has come off David Cameron."

The fears in Downing Street came as Lord Macdonald, the former director of public prosecutions, hit back at a suggestion by former attorney general Lord Goldsmith that he had questions to answer over the failure to widen the phone hacking inquiry in 2006. Macdonald said: "What you can't do in a case is have 100, 200 or 300 counts on the indictment. The judge just wouldn't stand for it. What you do – and this is conventional charging strategy – is you put sample counts on.

"So Clive Goodman [former NoW royal editor] and Glenn Mulcaire [private investigator hired by the NoW] were charged. What the prosecutor [Carmen Dowd] did was select four victims and each one was representative of a sub-group. So you had one member of the royal family, one celebrity, one sportsman, one actress. That count was representative of all the other stuff. These are called sample counts."

Macdonald, who stressed that Dowd had made the charging decisions, pointed out that his successor Keir Starmer told MPs this week that police had made clear in 2006 that no other journalist at the NoW, including Coulson, was linked to Mulcaire. Police gave this information to David Perry QC who reviewed the case for the Crown Prosecution Service.

Lies could lead to prison or fine

Imprisonment or a substantial fine could theoretically be imposed as a punishment by parliament on anyone who told lies in evidence to a select committee. Misleading MPs, or refusing to answer a summons to appear at a session, would be a "contempt of the House" and referred to the committee on standards and privileges. Any offender would be summoned to the bar of the Commons.

The problem is that the sanctions to enforce any punishment – fine or imprisonment – are constitutionally rusty. Vernon Bogdanor, the former professor of government at Oxford University, has suggested they may have fallen into "desuetude" [disuse]. The House of Commons is not believed to have fined anybody since 1666 and has not "committed anyone to custody", apart from temporarily detaining them, since the 19th century.

The last time the Commons attempted to reprimand anyone at the bar of the house was in 1957 when the Sunday Express editor John Junor was criticised after offending MPs by publishing an editorial accusing them of abusing their petrol allowances. "Such a sanction would now appear high-handed," a recent standard and privileges committee report acknowledged.

An alternative course of actionwould have been for a select committee to ask witnesses to take an oath before giving evidence. Lying under oath would be perjury. It would be up to the Commons, or the police, to administer any punishment – the range of which is similar to those for anyone deemed to be guilty of contempt. The culture, media and sport select committee is understood to have considered – but ruled out – this option before Tuesday's session.It is extraordinarily rare, however, for witnesses to be asked to swear in advance that what they are about to say was "the truth, the whole truth and nothing but the truth".

According to a recent edition of Erskine May, the encyclopedic handbook of parliamentary procedure, "witnesses who give false evidence, prevaricate, present forged or falsified documents to a committee with intent to deceive the committee ... or who are guilty of disrespectful conduct to the committee in a state of intoxication may be reported to the House."

The last time the House of Commons imprisoned someone who was not an MP, except overnight in the custody of the Serjeant at Arms for disorderly conduct in the galleries, was as long ago as 1880 (for failing to attend as a witness).

Owen Bowcott