But she could still face a re-election battle after Conservatives in her north London constituency said they were seeking to get her victory overturned by the High Court. One source said as many as 600 proxy ballot papers may have been involved.
Ms Jackson won the seat in April with a majority of 1,440 over the Tory candidate, Oliver Letwin.
Camden council called in Scotland Yard after two people complained that they arrived to vote at a polling station in the Hampstead and Highgate constituency only to be told someone had done so on their behalf by proxy.
Police said that forgery and counterfeiting had occurred, but Detective Sergeant Michael Corsie, the investigating Special Branch officer, said that there had been no breach of election law. He had not been able to identify those responsible, and the Crown Prosecution Service had recommended 'no further action'.
A spokesman for local Conservatives said that they were 'disappointed' by the CPS decision. Carl Gibson, Tory agent for Hampstead and Highgate, said: 'This is a very grave and serious matter. We are now considering applying to the High Court for a writ to overturn the result.
'We will be discussing with the Scotland Yard Fraud Squad the full implications of their findings to see how widespread this matter was. It must be appreciated that we held this seat for more than 20 years and we cannot allow Glenda Jackson to remain in office when the police have confirmation that such offences have taken place.'
A spokesman for the MP said last night: 'We understand from the police that no breach of the People's Representation Act has occurred and the police were not pursuing the matter any further.
'While we regard any suggestion of electoral malpractice as a serious offence, we have been told there was probably only one individual concerned and there is no real suggestion of the vote being overturned.'
For a High Court move to succeed it had to be shown a breach of the Act had occurred and that it had a material effect on the result.
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