By Dominic Casciani
The director of public prosecutions has announced that four parliamentarians will face criminal prosecution over their expenses claims.
In his statement, Keir Starmer QC underlined that the four had a right to a fair trial and that the question of whether parliamentary privilege would form part of the case.
So how and why has the issue of parliamentary privilege become a factor in the prosecutions?
What is parliamentary privilege?
Parliamentary privilege is one of the most ancient rights enshrined in British law and it has a very special place in British constitutional history.
In essence, parliamentary privilege developed as a means of stopping a monarch from interfering with the workings of Parliament.
It was enshrined in the 1689 Bill of Rights. That act came within living memory of the English Civil War, sparked by a power struggle between King Charles I and Parliament.
One of the rights in the bill is "that the freedom of speech and debates or proceedings in Parliament ought not to be impeached or questioned in any court or place out of Parliament."
So what does that mean in practice?
Parliamentarians have the right to say whatever they like in Parliament and they can never be sued for libel for doing so. It also means the public and media have a right to report what is said in Parliament.
Parliamentary privilege has expanded to cover the preparations MPs make for debates, such as their papers, tabled questions and their own notes. Draft statements submitted by witnesses to parliamentary inquiries are also protected.
What does the act mean when it refers to protecting "proceedings"?
A joint parliamentary committee examined this issue in 1999 and said that "proceedings in Parliament" needed to be clarified and defined.
"[The 1689 Act] protects activities that are recognisably part of the formal collegiate activities of Parliament," it said.
An even earlier committee said that "proceedings" should cover "all things done or written by a member" as part of their duties - or for the purpose of enabling them to do their duties.
So how could privilege apply to expenses?
The 1999 committee said that there was a clear modern interpretation of the 1689 act and its purpose. It said the law protected "members of both houses from being subjected to any penalty
in any court for what they have said in the course of the proceedings in Parliament."
The question for the prosecution will be whether the word "proceedings" in the original act can be interpreted as covering expenses claims submitted by a parliamentarian.
If it does, then the lawyers for the accused parliamentarians will further argue the four members should not face criminal trial over their expenses claims because the allegations should be dealt with by other MPs and peers rather than the criminal courts.
So is the law unclear?
The Crown Prosecution Service does not take a case to court unless it thinks there is sufficient evidence and a reasonable chance of a conviction.
In his statement on the prosecutions, Director of Public Prosecutions Keir Starmer QC said: "Lawyers representing those who have been charged have raised with us the question of parliamentary privilege.
"We have considered that question and concluded that the applicability and extent of any parliamentary privilege claimed should be tested in court."
In other words, he does not accept the argument - but it will need to be tested before a judge.
What's the expert view on this?
Erskine May, the official handbook on parliamentary practice, defines privilege as "the sum of peculiar rights
without which [MPs] could not discharge their functions".
The CPS has also made its own expert submission to the 1999 parliamentary inquiry.
It said: "The exact scope of the phrase 'proceedings in Parliament' [in relation to privilege] is far from clear. Accordingly, it is very difficult to assess the evidential sufficiency of any particular case.
"Clarification of the scope of the phrase would assist by facilitating a more accurate assessment of the evidence available to the crown in any individual case.
"Naturally, the narrower the scope of the phrase the less parliamentary documentation would be afforded the protection of parliamentary privilege and the easier it would be to prosecute corrupt conduct on the part of members acting in that capacity."
Has this situation ever come up before?
In November 2008, police raided the parliamentary office of Tory MP Damian Green in an investigation into leaks. A furious row blew up over whether the police had breached parliamentary privilege by entering the Palace of Westminster and seizing protected documents from Mr Green's office.
But the issue was never settled. The police investigation was dropped before there was an opportunity to have an argument in court over what was covered by privilege.
The Green affair led to the Speaker of the House of Commons clarifying the manner in which detectives can enter Parliament. It also prompted the Commons to set up a committee to examine how and when privilege covers a parliamentarian's correspondence and communications. Its report is due soon.
Are British parliamentarians immune from prosecution?
No. There is no law that prevents an MP or peer being prosecuted, unlike in some countries. The most recent case involved Labour's deputy leader Harriet Harman. She pleaded guilty earlier this year to driving without due care and attention.