Tony Blair was advised that he should use the “government machine” to push for a Yes vote in the referendum on the Good Friday Agreement – but not to the extent that it would risk calling the result of the historic vote into question.

As Northern Ireland’s parties inched towards signing the peace deal in April 1998, declassified state papers reveal that preparations were already well advanced for planning for the referendum which would follow.

The Good Friday Agreement, also known as the Belfast Agreement, was a political deal designed to bring an end to 30 years of violent conflict in Northern Ireland.

It was signed on April 10 1998 and approved the following month by public votes in Northern Ireland and the Republic of Ireland.

Just days earlier, on April 6, the prime minister’s cabinet secretary Richard Wilson wrote a memo to Mr Blair setting out what position government ministers should take if a deal was signed and a referendum called.

He said: “My judgement is that – once ministers have decided the prior question of whether campaigning for a ‘yes’ vote is the best course politically – they should go on public record with a statement explaining that they will be campaigning as government ministers and, as such, will continue to draw upon the support of the government machine, including the civil service, in pursuit of their policy objectives.

“I further recommend that the scale and nature of the support given to ministers should be carefully circumscribed… to ensure the playing field is not so tilted in favour of the ‘yes’ campaign as to call into question the validity of the result.”

Mr Wilson said there was a “respectable case for neutrality”, but then pointed out ministers had campaigned for Yes votes in devolution referendums in Scotland and Wales in 1997.

He added: “Having arranged for a referendum to take place, it is incumbent upon the government to ensure that the voters understand its position on the question.”

Mr Wilson then turned to the question of what support ministers should have from the “government machine”, including the civil service, when campaigning for a Yes vote.

Ministers should not tilt the scales to such an extent that the validity of the result could fairly be called into question

He said: “Ministers should not tilt the scales to such an extent that the validity of the result could fairly be called into question.

“At one end of the scale I would think this rules out an intensive television, radio and newspaper campaign paid for out of public funds.

“At the other end of the scale, I see absolutely no difficulty in ministers who are campaigning for the endorsement of government policy drawing upon the backing of the government machine – for example in terms of official transport during the campaign, and the production by civil servants, including government information officers, of briefing, speaking notes and material in response to questions about and criticisms of government policy.

“In providing such back-up, I think particular care will be needed to ensure that material produced is not partisan, unfair or unreasonable.

“While ministers will be provided with more official support than during those campaigning in the Scottish, Welsh or London referendums, I would advise that the NIO follow the prudent course of the Scots and Welsh in ensuring that its public statements about the Agreement and referendum are ‘proofed’ by lawyers to reduce the Governments susceptibility to political and legal challenge through judicial review.”

Explaining why ministers would take a more active approach in the Northern Ireland referendum than they had done in Scotland or Wales, Mr Wilson said it could be justified because the vote was a “result of a bipartisan policy pursued by the main political parties of the UK”.

He added: “This seems to me to be a defensible position.”

The state papers reveal that the following day a civil service memo entitled “Political Agreement: Planning an Announcement and Publication” was sent.

It said that after a political agreement was signed, the text should be sent to key individuals in Northern Ireland, Great Britain, the US and Europe along with a covering note and a phone call from Mr Blair or secretary of state Mo Mowlam.

The memo said the Government should avoid “active campaigning” during the referendum campaign, leaving this instead to the Northern Ireland political parties.

However, while the referendum planning continued, it was still not clear at that point that the parties would reach political agreement.

A note attached to the civil service memo on April 7 is entitled: “Political update – Grim Outlook (Bridges Too Far?)”

It stated: “Things have taken a serious turn for the worse with regards to the political process.

“At approximately 2:30pm today the unionists issued text of a letter to the Irish Government and to the Prime Minister which vehemently rejected Senator (George) Mitchell’s synthesis paper.

“Whilst not talking in terms of a walk-out, they have signalled that radical changes are required.

“Tony Blair is flying in at 4:40pm and going straight to Hillsborough. It is planned he will have a meeting at 5:30pm with David Trimble.

“The way forward is completely unknown.”