JIM Callaghan's Labour government considered a referendum on Scottish independence as part of its constitutional reform package in the 1970s.

Ministers were urged to "meet the SNP challenge head on" by offering, and defeating, separation in a poll that included the option of devolution. But the plan was shelved after Callaghan, then the prime minister, said English voters would have to be included in any plebiscite on breaking up the UK.

The discussions are contained in government papers released by the National Archives. The files cover 1977, the year when the Labour government was putting together its plan for a referendum on a Scottish assembly.

Papers show a debate took place within the government on whether independence should also be put on the ballot in order to defeat it.

A meeting took place in February 1977 between Callaghan, his officials, junior minister John Smith, and Labour MPs Robin Cook and Norman Buchan, in which the idea was studied in detail.

Buchan, an advocate of the plan, was noted in the minutes as saying an independence referendum would help "settle the issue", adding: "There was now a public expectation that there would and should be an independence question."

He is also recorded as saying: "The Labour Party needed an opportunity to meet the SNP challenge head on."

However, the records show Callaghan was sceptical. He said an independence question could not just be voted on by Scots, but that the English were also "entitled to vote on the dissolution of the UK".

The minutes also said: "The prime minister said that the more Cabinet had looked at the implications of the English aspect of the vote on independence, the more overwhelming their view against had been."

Callaghan concluded the meeting by stating his opposition to the plan: "The UK government could not be seen to put to one part of the UK a question about its break-up in the absence of any real demand."

The files also reveal the civil service's resistance to a multi-option poll on constitutional change.

Michael Quinlan, then a key figure at the Cabinet office, criticised the idea in a briefing note ahead of the summit. Although he predicted an independence referendum would produce a "big majority for staying in the UK", he laid out several reasons for not including separation on the ballot paper.

He believed an independence question would mean "giving the SNP the limelight", rather than the government's devolution plan, as well as reinforcing the view that "devolution and separation were somehow connected".

And he also wrote: "The SNP might secure an inflated vote for separation by putting across a glamorised, having-it-both-ways picture of what it entails."

Another Quinlan memo made a similar argument for rejecting an independence referendum, stating: "There might be a graver risk of comparatively irresponsible voting, or voting directed to tactical objections. This might be particularly high, and the result particularly damaging, in relation to a question about Scottish independence."

The current SNP Holyrood government's policy is to stage a referendum on independence before 2011, a plebiscite first minister Alex Salmond believes could include other options, including more powers for Holyrood.

An SNP spokesman said: "Thirty years ago, those opposed to independence and equality for Scotland ultimately didn't have the confidence to put it to the people in a referendum - and it is clear that nothing has changed since.

"The recent poll in the Sunday Herald showed support for independence at 40% and on a rising curve - much higher than in the 1970s. The big difference is we now have a Scottish parliament and government with the ability and initiative to take Scotland forward."