Russian spy claims: How MPs' staff are vetted
It has been likened to an episode of espionage drama Spooks.
The news that a Russian woman who has been working in the House of Commons faces possible deportation, amid newspaper reports that she is a spy, has been greeted with disbelief by many MPs.
Lib Dem MP Mike Hancock, who employed 25-year old Katia Zatuliveter as a parliamentary assistant, says there is "no evidence" that she presented a threat to national security and she would appeal.
The MP, who is a member of the defence select committee, points out that the security services had the opportunity to remove her when they interviewed her during the summer but chose not to.
He has made clear that Ms Zatuliveter graduated from a British university before initially working as an intern at the Commons and her sister is married to a UK citizen.
But the case does raise questions about how parliamentarians vet their staff - thousands of whom have daily access to MPs and sometimes ministers - and how watertight the procedures are.
Anyone wanting to work for an MP, either in their parliamentary or constituency office, has to get security clearance from the Home Office.
Applicants have to undergo an enhanced criminal records check.
However, unlike in some other public sector fields, the process is not limited to finding out whether someone has been convicted of any criminal offence.
It also seeks to identify any risks stemming from someone's past or family background.
The kind of questions it will ask are details about an individual's parents and relatives and what kind of political activity they have been involved in.
It is designed to sift out anyone who has a personal or family history of involvement with extreme political groups or causes.
The vetting process for foreign nationals, and those outside the EU, generally takes longer.
Those given approval to work in Parliament - both in the Commons and Lords - normally get an initial nine-month clearance. They will then be expected to reapply for clearance every two years.
One security expert said suggestions that MI5 had sought the deportation of a Commons researcher was a "very serious" matter for Parliament.
"In recent years MI5 has tried to keep well away from the House of Commons," Professor Anthony Glees, Director of the Centre for Security and Intelligence at the University of Buckingham, told the BBC.
"It is worried what MPs will say if it looks too closely at their affairs.
"But, in fact, the director general of MI5 and the chief of the Secret Intelligence Service [MI6] have, in public, warned precisely against this danger," he added.
After last year's expenses scandal in Westminster, one suggestion floated to improve the system of staff support was for researchers, secretaries and other helpers to be hired and employed centrally by the House of Commons, not by individual MPs.
This was eventually rejected as impractical and many MPs - who have relied for years on support from their wives/husbands and family members to run their offices - felt that it represented an assault on their freedom to organise their own affairs.
But the Committee for Standards in Public Life, whose blueprint for expenses reform was largely adopted by Parliament, recommended there should be "binding guidance" for MPs when recruiting staff.
It called for a code of conduct for MPs' employees, setting out "appropriate restrictions" on party political activities they could undertake.
It also said MPs should be responsible for ensuring their staff abide by this code and sign an annual declaration attesting to having done so.
Commons security is ultimately the responsibility of the House of Commons Commission, which oversees Parliamentary buildings, chaired by Speaker John Bercow.