The House of Commons office of Damian Green, the Tories' immigration spokesman, is routinely swept for electronic bugging devices, along with other offices belonging to senior Conservatives, amid fears of covert monitoring, The Independent on Sunday has discovered.

Anger surrounding the shadow immigration minister's arrest last week escalated dramatically last night over suspicions of a major bugging scandal inside the Palace of Westminster.

The IoS understands that even before his surprise arrest on Thursday Mr Green was aware that his Commons office, phone calls and emails could be under surveillance because of the sensitive nature of his job.

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The fresh revelations rocked the Commons just days before the high point of the parliamentary calendar, the Queen's Speech, which takes place on Wednesday.

Tory leader David Cameron last night said the Prime Minister must denounce the arrest of Mr Green or risk charges of hypocrisy because he "made his career" from Whitehall leaks. Writing in the News of the World, Mr Cameron added: "If this approach had been in place in the 1990s, then Gordon Brown would have spent most of his time under arrest."

Several offices within the Commons and Portcullis House belonging to senior Tory MPs and officials are checked regularly by security experts for listening devices and other surveillance equipment.

The IoS has learnt that there are "major concerns" at the highest levels of the Tory party over suspected monitoring by the authorities. Any such monitoring may not be illegal but would be hugely controversial.

Last night, a Conservative MP wrote to Gordon Brown demanding an urgent review of the Wilson doctrine, the convention that protects MPs from phonetapping but does not cover other surveillance techniques.

It is not known whether a covert device has ever been found during searches. But if the suspicions are proved right, it would have major implications for the protection of parliamentary privilege.

Ben Wallace, the Conservative MP for Lancaster & Wyre, said the Wilson doctrine, which dates back to 1966, needed to be changed to cover all forms of surveillance, not just intercepting of calls.

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He said: "It is disturbing that the authorities may have exploited the difference between surveillance and intercept in order to pursue Members of Parliament over the past 10 years."

Mr Green's arrest and detention for nine hours, and simultaneous raids on his office in Portcullis House, constituency office and home over the Whitehall leak investigation has already triggered a major political row. David Cameron, the Liberal Democrat leader Nick Clegg, and MPs from all parties have reacted furiously to the treatment of the MP for Ashford.

It is understood that disquiet at the police action, in particular the approval by the Serjeant at Arms to allow anti-terrorist police to raid a parliamentary office, has even reached cabinet levels, but Downing Street maintains the investigation is a police matter. Ministers have insisted that the Prime Minister and the Home Secretary, Jacqui Smith, did not know in advance that Mr Green was to be arrested.

Mr Green, whose mobile phone was taken by police as part of their investigation into leaked stories from a Home Office civil servant, was forced to communicate through his Facebook page yesterday.

A party spokesman refused to comment on the revelations last night, as did Mr Green.

There were reports that the civil servant accused of leaking stories is an assistant private secretary who was temporarily attached to the special advisers' office at the Home Office.

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