Downing Street has been accused of telling "whopping untruths" over the publication of a report into the threat posed by Russia to the UK's democratic processes.
The claim by former attorney general Dominic Grieve ramps up the war of words with Number 10 over the release of the dossier, which he argues is essential ahead of the election.
Labour leader Jeremy Corbyn has also demanded its publication, questioning what the government "have got to hide".
Mr Grieve, the chair of parliament's Intelligence and Security Committee (ISC), alleges the prime minister is sitting on the report ahead of the 12 December poll.
The independent MP who was kicked out of the Tories by Boris Johnson over his opposition to no-deal Brexit is calling on the Conservative leader to publish the committee's report before parliament is dissolved on Tuesday.
The House of Commons has previously been told the report was sent to the PM on 17 October.
Mr Grieve says Downing Street should have signed off the report within 10 working days, but No 10 dispute this and maintain the process usually takes six weeks.
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Parliament is expected to sit on Monday and Tuesday ahead of dissolution although proceedings could be brought to an end sooner, which may prevent the report's release.
Speaking to Sky News, Mr Grieve said: "If it can't be published by Tuesday it will not be published at the earliest until April or May of next year.
"It does have a bearing on electoral processes because we know that the Russians have in other countries interfered in their electoral processes."
He said that Downing Street's six-week claim on clearing the report was "completely and totally untrue".
Mr Grieve added: "It's a lie. I really get worried about this. The process of getting this report cleared is finished.
"The last stage – the clearance by the prime minister – is programmed for 10 days.
"The suggestion that six weeks are needed is just astonishing.
"They are coming out with things that are quite simply whopping untruths and they must know it."
He went on: "I find it really worrying. WhaRead More – Source