The row caused by the arrest of a Tory MP for passing Home Office leaks to the press intensified today after Jacqui Smith denied the government was resorting to "Stalinism" in order to silence dissent.

The Home Secretary told the BBC that officers had to be allowed to “follow the evidence where they need to”, and denied that she had been informed by police in advance about plans to arrest and interrogate Damian Green.

However, in a statement which appeared to signal some divisions within the government on the matter, Ms Smith's Cabinet colleague Harriet Harman later said she was "very concerned" about the arrest and announced that Speaker Michael Martin would investigate the procedures by which police are able to enter the Palace of Westminster.

Mr Green was held and questioned for nine hours on Thursday “on suspicion of conspiring to commit misconduct in a public office and aiding and abetting, counselling or procuring misconduct in a public office”.

His homes and offices, including in the House of Commons, were searched and his computer, phone and other communications equipment seized in the raids.

It followed the earlier arrest and then suspension of a junior Home Office official, believed to be 26-year-old Chris Galley.

Speaking on BBC1's Andrew Marr Show, Ms Smith defended the investigation, saying that it was examining a “systematic series of leaks” of potentially sensitive material beyond the cases being claimed by critics.

“It is not an investigation into whether or not Opposition politicians used information they received to embarrass or hold to account the Government,” she said.

“That is a completely legitimate activity it has gone on; it should go on; it will go on.

“This started as an investigation of a systematic series of leaks from a department that deals with some of the most sensitive and confidential information in government.

“There are four leaks that are in the public arena. The point is that this started as an investigation into a systematic series of leaks about which, of course, it was not clear what had been leaked and what may not have been leaked.”

Repeating her insistence that she had not been aware a senior Tory was to be arrested until after it happened, she said: “What we appear to be being asked to do, by former Home Secretaries, by the Leader of the Opposition, is to intervene in a specific investigation being carried out by the police who I do believe, when they start an investigation, should, as they have said they need to, follow the evidence where they need to."

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