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), said the committee.
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.
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."