A "yes" vote in next year’s Scottish independence referendum would damage Britain’s international reputation and incur unforseen costs for a new state, a committee of members of Parliament has concluded.

The influential Commons Foreign Affairs Committee said that too many assumptions were also being made by the current nationalist government in Edinburgh on areas such as EU membership and its own state intelligence.

The MPs said Britain’s Foreign and Commonwealth Office was not doing enough to limit any potential impact of a yes vote in the referendum, which takes place in September 2014. UK ministers have held back from discussing post-independence terms and agreements, possibly prompted by various opinion polls suggesting Scots will vote no.

The new report, however, entitled "Foreign Policy Considerations For The UK And Scotland In The Event Of Scotland Becoming An Independent Country," said there were too many remaining questions, such as the future of Britain’s Trident nuclear deterrent, which is based in Scotland; future Scottish EU membership; and diplomatic missions.

It states: “It is difficult to measure the impact on the RUK's [rest of the UK] international standing and influence in the event of Scotland becoming an independent country but we conclude that some degree of reputational damage is inevitable.”

Deputy First Minister Nicola Sturgeon of the Scottish National Party (SNP), which has pledged to remove Trident, welcomed parts of the report but said it was written by a "partisan" group opposed to independence.

She added: “We have continued to press the UK Government to engage in constructive discussions and sharing of information prior to the referendum so that voters are as informed as possible about what happens next.”

Professor Robert Hazell, director of the Constitution Unit at University College London, said the UK government was justified in not doing so. “I’ve always said that this is [Scottish First Minister] Alex Salmond and the SNP’s project, so let them do the running by explaining what it will mean and the benefits. Whenever he [Salmond] is asked about the detail, he always uses his default position: ‘oh we’ll negotiate an agreement with the rest of the UK about the pound, power generation,’ and the like, but he’s coming unstuck on that.

“I don’t think the Scottish people do understand the consequences leaving the UK will have on foreign policy. It’s being portrayed abroad as some internal, family row, but it’s not – the UK is a leading member of the EU, NATO, of which Scotland is currently part, so it will have a big impact.”

Professor Hazell says he understood why Prime Minister David Cameron’s government was not entering a full debate. He adds: “It’s difficult for the UK government because all the forecasts are fairly strong that the Scots are not going to vote for independence, so why should they spend already scarce resources making contingency plans? They also do not want to be seen ‘greasing the slipway.’ ”

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Adam Hug, policy director at the Foreign Policy Centre, said there was lively debate in Scotland about independence but he had not noticed much about the potential effect south of the border. “Clearly there will be a big impact economically and politically if Scotland votes for independence. In Europe, Scotland is seen as a pro-EU influence within the UK, particularly when compared to a potential England-dominated rest of the UK.

“Similarly, other states in Europe, like Spain with their Catalan and Basque independence movements, are keen to maintain their territorial integrity and so are looking at what happens.”