PRO-Brexit MPs have stepped up their demand to have a pro-EU UK Government website taken offline, warning that ministers and civil servants would be breaking the law if it remained in place.

Leading Leave campaigner Boris Johnson is among 28 MPs who have written to Cabinet Secretary Sir Jeremy Heywood calling for him to withdraw the pages on the Gov.uk website when the so-called purdah period comes into force on Friday.

The Electoral Commission has advised that the website can legally stay up so long as it is no longer updated or linked to and "members of the public need to take active steps to seek access to the content".

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But the MPs said legal opinions from Speaker's Counsel Michael Carpenter suggested the current plans did risk breaching the rules requiring Whitehall to withdraw from campaigning in the closing weeks.

"We therefore demand that any government web pages carrying material published with the intention of persuading people to vote remain should be taken down before Section 125 of PPERA (Political Parties, Elections and Referendums Act 2000) comes into force this Friday," they wrote in a letter copied to David Cameron.

"Failure to carry this out will put both ministers and civil servants in breach of the law, of the Ministerial Code and of the Civil Service Code. Please can you reassure us that you will act," they concluded, demanding to know if disgruntled senior civil servants had made any formal protests over the decision to maintain the site.

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The Prime Minister was previously warned by the pro-Brexit Bernard Jenkin, who chairs the Commons public administration and constitutional affairs committee, to "expect a writ" if no action was taken.

Mr Cameron told him calls for information to be taken down were an "extreme position".

Meantime, standing in for his cabinet colleague at Prime Minister’s Questions – the PM is at the G7 summit in Japan - George Osborne said the UK Government was "confident" it was acting within the law and accused Leave campaigners of focusing on the process of the referendum rather than the issues.

The Chancellor was challenged by Mr Jenkin, who told him the committee had published three legal opinions from the Speaker's Counsel that "make it perfectly clear it is illegal".

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Mr Osborne told him: "Of course, the Government will comply with the law and Government websites will comply with the purdah rules and we are confident that they do.

"But can I make a general observation. You and I have fought for this referendum, the referendum is taking place, there are some huge issues at stake about Britain's economy, Britain's security, Britain's place in the world.

"We have perfectly honourable disagreements on those big issues. Let's debate the substance rather than the process and then the British people will feel they've had a range of opinions and they can make their mind up," added the Chancellor.

Later, a Downing Street source said: "We will obviously keep within the rules and the rules are pretty clear. The Government has a clear position on this referendum, that's why we put things on websites, produce leaflets and so on.

"But, clearly, when you come into the purdah period there will be no new content added to any of those websites."

Asked if content already on the sites would be taken down, the source said "you can't unpublish" what has already been posted online. "There wouldn't be any links or any new content," the source added.