UK referendum rules could 'hobble' government

Purdah is a traditional practice observed in the last four weeks before national elections in the UK, during which ministers and the civil service are forbidden from issuing political statements.

"In the last 28 days of the referendum campaign, let's assume there are 10, 11, 12 of councils [of European ministers]. Some of those will be informal and private, others will be formal ministerial councils in which legislation is negotiated. British ministers and sometimes British civil servants ... will be sitting in those councils trying to secure the best outcome for Britain", Hayward said.

“We need to make sure ... that our ministers use whichever are the most potent arguments they can use to win points for Britain in those negotiations".

Plans by UK PM David Cameron’s government to scrap the purdah have already stoked controversy among eurosceptics in his Conservative party, who fear the government will rig the referendum in favour of a Yes vote to continue EU membership.

Twenty seven Conservative MPs, mostly from the party’s most eurosceptic wing, voted against the government’s plans when its EU reform bill went through its second reading in the House of Commons in June.

However, Hayward expressed concern that unless the rules were eased, ministers could put themselves at risk of legal action.

“Our legal advice is very worrying on this aspect, that unless ministers tread very carefully they may well end up using arguments in those internal EU discussions which could be construed by anyone who is litigious as bearing on those questions of the referendum", he said.

"The sharpest case would be where a minister is negotiating on a highly contentious issue which everybody in this committee would agree was vital to the UK national interest, and felt hobbled by being unable to make any points about the importance of this to the referendum and so on".

Debate in the UK parliament will resume in the autumn, when the government has promised to table amendments to the bill that would “put beyond any doubt that the campaign will be conducted throughout in a manner that all sides will see as fair”, according to Europe minister David Lidington.

Cameron set out his wishlist at an EU summit in June, demanding the removal of the phrase “ever-closer Union” from the treaty, curbs on welfare for EU migrants, and safeguards for the UK as a country outside the eurozone.

He has conceded that the referendum - which is likely to be held in autumn 2016 or early 2017 - will be held without a reform package having been ratified by other EU governments.