SNP ministers will be bound by law to cease campaigning for independence 28 days before the referendum but UK ministers can legally promote the Union until the polls close, according to experts.

The Scottish Government could expose itself to a judicial review if it makes any pronouncement related to the referendum during this period, whereas the UK Government would merely be bound by a "gentleman's agreement" to stay silent, Holyrood's Referendum Bill Committee heard today.

The 28-day rule applies to any part of the Scottish administration, the Scottish Parliament Corporate Body and any Scottish public authority but does not cover the UK Government or authorities.

The UK Government made a commitment to observe the 28-day rule in the Edinburgh Agreement last year but any fallout for breaching this "would be more political than legal", according to Law Society of Scotland director of law reform Michael Clancy.

In a submission to the committee, the Law Society questioned whether the UK Government would follow the same "purdah" (period of silence) rules set out in the Bill.

SNP MSP Linda Fabiani said: "Would that be a veiled suggestion that we should check up on this and perhaps, as a committee, get this in writing from the UK Government?"

The Edinburgh agreement set out the conduct of both governments during the referendum and "is very much a mutual respect and gentleman's agreement", she said.

Mr Clancy said: "The Edinburgh agreement does contain this (28-day) provision and it's mentioned in the policy memorandum, so one can take that as being: if it's under the signature of the Prime Minister with the First Minister in attendance and signing too then people understand what's what.

"I think there is distinction between a statutory provision and something in an extra statutory agreement which people may want to flesh out."

Speaking after the committee, Mr Clancy said that "there would be a breach in the law" if the Scottish Government breaks the 28-day rule.

"It means that the Scottish Government would be subject to judicial review for any breach of the statutory duty for what it had done in that respect," he said.

"The court would be able to make an order requiring rectification of that but what it would mean in terms of this actual Bill I couldn't predict.

"The UK Government has to act reasonably but there would be no statutory obligation on it so the consequences would be more political than legal."

The committee also heard from a professor of politics and people from both sides of the 2011 referendum on changing the UK election system.

MSPs were advised that purdah should be extended in the terms of the referendum legislation.

William Norton, referendum agent in the No to AV campaign, said weaknesses in UK legislation have been carried over, and one significant element has been missed out from the 28-day rule.

"In the original, it would have included other organisations that aren't public bodies which are majority funded by the taxpayer," he said.

It was either a mistake or "someone took a decision to leave it out", he told the committee.

The implication is that Government-funded bodies could carry on.

Mr Norton, who said the 28-day rule is too short, suggested it should be extended to a point already passed in the Scottish referendum campaign.

"I would limit it from the day one which designation occurs or doesn't occur," he said.

"Designation is effectively the date on which publicly recognised campaigners come into being."

Professor Richard Wyn Jones, professor of Welsh politics and director of the Wales Governance Centre at Cardiff University, insisted the rules must apply to both sides.

"In my view, that would be a resounding yes to that," he said.

A spokeswoman for the Electoral Commission in Scotland confirmed that the UK Government will not be bound by the 28-day rule in the Referendum Bill, nor will it be bound by similar prohibitions that regulate national elections.

The 28-day rule in the Referendum Bill has been adapted from a similar restriction in the UK Political Parties, Elections and Referendums Act 2000 (PPERA) which regulates UK-wide polls.

Scotland's referendum will be regulated by the Scottish Independence Referendum Bill, not PPERA, meaning UK Government pronouncements will be exempt from both, the commission said.

PPERA was drafted in 2000 partly in response to concerns about UK Government "propaganda" during the 1997 devolution referendum.

The then Labour UK Government was campaigning for a Yes vote and dispatched a document to every household setting out the Government's views.

The Neill Report on Standards in Public Life, which inspired PPERA, said: "The Government's document claimed to be doing no more than providing the voters with unbiased factual information.

"The document was nevertheless criticised on the ground that, merely by summarising the Government's White Paper on Scottish devolution, it constituted, in effect, propaganda for the Yes side."

The report concluded: "The government of the day in future referendums should, as a government, remain neutral and should not distribute at public expense literature, even purportedly 'factual' literature, setting out or otherwise promoting its case."

In a statement released after the committee meeting, Ms Fabiani said: "The Edinburgh agreement firmly commits the Westminster Government to observe the same purdah arrangements as the Scottish Government, and people in Scotland will rightly expect Westminster to stick to those terms.

"Any attempt by Westminster to violate the terms of that agreement would be a gross misjudgment and a clear breach of their responsibility to help ensure that the referendum is held in a fair way."