Scottish independence would "inevitably" damage the UK's international reputation and the Foreign Office must take action before the referendum to limit the potential impact, a committee of MPs has said.
The UK Government's confident assertion that Scotland will vote No is causing a problem for foreign partners, according to the Commons Foreign Affairs Committee.
UK ministers frequently state that they will not pre-negotiate the terms of independence and that they have not done any substantial contingency planning in the event of a Yes vote.
But this has led to the perception abroad that Whitehall is largely unresponsive to calls for more information about what independence would mean for the remainder of the UK (RUK), the committee said.
The Scottish Government has been asked to provide more evidence for its assertions that Scotland will gain seamless membership of international organisations such the European Union and Nato without losing the UK's existing benefits.
Scotland could also be forced to keep nuclear weapons for a generation, according to the committee.
Pressure from international partners and post-independence negotiations over Scotland's share of the UK's assets and liabilities could limit the SNP's ambition for a speedy removal of Trident, it said.
The advice is contained in a new report entitled Foreign Policy Considerations For The UK And Scotland In The Event Of Scotland Becoming An Independent Country.
"It is difficult to measure the impact on the RUK's international standing and influence in the event of Scotland becoming an independent country but we conclude that some degree of reputational damage is inevitable," it states.
"We recommend that ahead of the referendum, the FCO (Foreign and Commonwealth Office) does more, when appropriate, to engage with international partners in order to highlight the UK's commitment to a consensual and broad-based engagement on the Scottish referendum, with a view to minimising the risk of damage to the UK's reputation.
"Witnesses did agree that the way in which the RUK handled 'the business of the break-up' would significantly influence how much reputational damage and loss of prestige the RUK suffered internationally.
"We heard from a number of witnesses that proactive political and diplomatic management of the situation would be required to prevent objections arising from key states.
"Thus far, it is not clear that the UK is doing this, partly because it has chosen to state, as a policy position, that it is confident Scots will vote to stay as part of the UK.
The report adds: "The problem with this strategy from an international perspective, as Catarina Tully observed, is that 'countries are finding it difficult to assess their own response to independence since they are not getting much response from Whitehall'. They are uncertain about what the Scottish referendum means and what Scottish independence might mean."
The committee also calls for "more candour" from the Scottish Government about what people in Scotland would lose and what it could realistically deliver, in foreign policy terms.
"We are concerned that seemingly unfounded assertions and initial negotiating positions are being presented as incontrovertible facts and that legal positions are being advocated without the benefit of official legal advice," it states.
The SNP's commitment to removing Trident from Scotland is likely to have "a significant effect on the willingness of the UK to co-operate on other issues upon which Scotland may need assistance, as well as influencing its overall position on the independence settlement".
While UK ministers say they are committed to maintaining Trident, whatever the cost of relocation, committee witnesses said the costs may be so prohibitive that a nuclear-free Scotland could precipitate full UK disarmament.
"Any resulting disarmament by the RUK would be received badly by the UK's key allies and could create problems for Scotland with other Nato and EU members as it forged a path as a new state," the report states.
"While the Scottish Government's commitment to removing nuclear weapons is not in question, international factors may constrain its ability to realise its goal and could mean that Scotland might not be nuclear-free for another generation."
Committee chairman Richard Ottaway, Conservative MP for Croydon South, said: "At the moment, there are some quite worrying gaps in the Scottish Government's foreign policy vision and certain assumptions are being made which don't seem to be based on concrete evidence.
"No one is doubting that Scotland could be a fully-fledged member of the international community but there is a real and urgent need for more information about the risks and costs involved.
"It is not enough to hope and assert that things will go in Scotland's favour at an international level and that goodwill will be forthcoming.
"A full and candid debate on the international implications of Scottish independence is now needed so that Scottish voters have the full facts before them when they vote in the referendum.
"We have raised a number of issues that we think deserve further scrutiny and which the Scottish and UK Governments must engage with in the coming months."
Deputy First Minister Nicola Sturgeon said the report had "some constructive aspects", including a suggestion that UN membership is likely to be "swift and unproblematic", that Scotland "could play a valuable role in Europe", and that it could in principle set up a successful diplomatic service.
She said: "These are rare examples of balance in a report that is clearly written from the partisan perspective of a group inherently opposed to the concept of independence and primarily concerned with protecting the interests, what it sees as the status, the prestige and crucially the pro-Trident position of the UK."
Ms Sturgeon agreed with the committee's call for the UK Government to engage more proactively with the referendum debate.
"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," she said.
"The Electoral Commission have also made recommendations to this effect. Yet the UK Government chooses to focus on the negative case for maintaining the status quo while steadfastly refusing to indicate that it will conduct sensible and consensual negotiations in the event of a vote for independence."Suggest a correction