Britain's international reputation will be damaged if Scotland becomes independent in a referendum next year, a powerful committee of lawmakers warned in a report published on Wednesday.
A split with Scotland would reduce Britain's influence in the European Union and reinforce the view that it is a world power in irreversible decline, the House of Commons Foreign Affairs Committee said.
The Scottish government described the report as "partisan" and said the committee only comprised members who are opposed to Scottish independence.
The United Kingdom is currently a permanent UN Security Council member, a nuclear-armed NATO nation and the third largest economy in the EU.
The report said it was "difficult to measure" the impact on the international standing of the remainder of the United Kingdom if Scotland became independent, "but we conclude that some degree of reputational damage is inevitable."
The lawmakers urged the Foreign Office to do more now to engage with international partners ahead of the referendum "with a view to minimising the risk of damage to the UK's reputation".
The report backed previous warnings by the British government that an independent Scotland would probably have to reapply for membership of international organisations such as the UN, EU and NATO.
"It is not in the gift of either Scottish or UK politicians to determine this unilaterally," it said.
But it said that the rest of Britain could also suffer a loss of both "hard" and "soft" power.
Doubts over the future of Britain's submarine nuclear deterrent, which is based in Scotland, could harm Britain's relationship with the United States.
They would "serve to fuel the views of some states that the RUK (remainder of UK) was a power in irreversible decline."
The committee agreed however that the way that Britain handled any break-up of the 300-year-old union with Scotland would "significantly influence how much reputational damage and loss of prestige the RUK suffered internationally".
The committee's report comes after a series of warnings from the government in London on the supposed dangers of Scottish independence.
Scotland's Deputy First Minister Nicola Sturgeon said the report lacked balance and accused the lawmakers of taking a negative approach..
She said it was "clearly written from the partisan perspective of a group inherently opposed to the concept of independence and primarily concerned with protecting the interests."
Sturgeon said that both the British government and the committee were "primarily concerned with maintaining the UK's international reputation and protecting its position as a nuclear state".
A referendum is set for September 18, 2014, when Scottish voters will be asked the yes/no question: "Should Scotland be an independent country?"