Downing Street has dismissed as not "credible or sensible" a proposal to designate the Faslane base, which hosts Britain's Trident nuclear deterrent, as sovereign United Kingdom territory if the people of Scotland vote for independence in next year's referendum.

No 10's dismissal of the idea followed an overnight row in Whitehall after the Guardian reported that the government was examining plans to ensure that the Faslane base on Gare Loch in Argyll and Bute could have the same status as the British sovereign military bases in Cyprus in the event of a yes vote.

The Guardian report, which came as the Scottish MP and former Labour chancellor Alistair Darling sets out the case for Scotland remaining within the UK, prompted the Scottish National party (SNP) to accuse London of seeking to bully Edinburgh.

A No 10 spokesman said: "This government has not commissioned contingency plans over Faslane. No such ideas have come to the secretary of state or the prime minister. They would not support them if they did. It is not a credible or sensible idea."

The Guardian said on Wednesday that Ministry of Defence officials were starting to examine plans to ensure that the Vanguard submarines could remain at the deep-water Faslane base and nuclear warheads could continue to be stored at the nearby Coulport base on Loch Long.

One source close to a senior cabinet minister said of the MoD's interest in the sovereign base idea: "What a ridiculous thing to say. Talk about handing a gift on a plate to the SNP."

The Guardian reported that, ahead of next year's referendum, the MoD was officially working on only one option for the Faslane base – a defeat for the SNP. An MoD spokesperson told the Guardian: "No contingency plans are being made to move Trident out of Scotland. The scale and cost of any potential relocation away from Faslane would be enormous."

But a defence source said that the idea of designating Faslane as sovereign UK territory in the event of an SNP victory was being taken seriously. The source said: "It would cost a huge amount of money, running into tens of billions of pounds, to decommission Faslane. Those costs would be factored into any negotiations on an independence settlement. The sovereign base area is an option. It is an interesting idea because the costs of moving out of Faslane are eyewateringly high."

A version of this was emailed to the BBC, which ran a story on its website overnight with the headline: "Faslane Trident base could be in UK after Scottish independence". The MoD emailed the BBC to say: "The sovereign base area is an option. It is an interesting idea."

Nicola Sturgeon, the SNP's deputy first minister, said: "Downing Street may now be denying this preposterous threat from the Ministry of Defence, but the cat is out of the bag that the UK government must at least be examining contingencies for a yes vote.

"Instead of dreaming up madcap scenarios in Whitehall bunkers to hang on to part of an independent Scotland, the UK government should sit down with the Scottish government in the light of day to discuss the range of issues that will require to be negotiated if Scotland votes yes – including removing Trident from Scotland as quickly as possible.

"The no campaign is in disarray – the MoD briefed an outrageous story about bullying Scotland into keeping Trident on the very day that Alistair Darling tries to pretend the no campaign isn't negative, and David Cameron is forced into denying something that has come out of his own government.

"The no campaign are running scared of their own Project Fear – and people in Scotland are beginning to look and laugh at the absurdities that are coming out of it. This latest episode has left the No campaign looking ridiculous – and it confirms that only a Yes vote next September will empower Scotland to get rid of Trident, and the money saved help build a fairer society and stronger economy."