'I would have thought they would become a London bank which would be symbolically quite important,' says business secretary

Royal Bank of Scotland's future as an Edinburgh-based bank was thrown into doubt after Vince Cable said it was inevitable the bailed-out bank would relocate to London in the wake of a vote for an independent Scotland.

The outspoken remarks by the business secretary – who also insisted that Scotland would need its own currency – raised fears about 3,000 jobs in the bank's lavish Gogarburn headquarters on the outskirts of Edinburgh and prompted an unnamed senior RBS banker to tell the Financial Times it was inconceivable that the bank would remain based in Scotland if the country voted for independence in September.

Officially RBS, a pillar of the Scottish business community, maintained that on the issue of independence it was politically neutral, but it refused to deny that the bank would abandon its Scottish roots after 300 years.

"We don't support political parties or political movements. We will respond to what voters decide and governments agree," a spokesman for the bank, 81%-owned by the taxpayer, said.

But the senior banker supported Cable's remarks to MPs on the Commons business committee that RBS would move to London because only the UK would be strong enough to support a bank whose balance sheet is almost the same size as the UK economy and required a £45bn bailout.

"If you were managing RBS, I think you would almost certainly want to be in a domicile where your bank is protected against the risk of collapse," Cable said.

"They already have a substantial amount of their management in London and I would have thought, inevitably, they would become a London bank which would be symbolically quite important."

The decision would rest with the RBS board but could also involve intervention by the UK government if it still holds its stake in the bank in 2016 – the year Scotland is expected to declare independence if there is a yes vote.

Cable also said it was almost certain that an independent Scotland would need its own currency – the latest intervention following remarks by Mark Carney, the governor of the Bank of England, the chancellor George Osborne, and Bob Dudley, the chief executive of oil group BP, on the difficulties of agreeing a new currency union should Scotland vote for independence in September.

The business secretary told MPs that the problems setting up a viable sterling currency union between Scotland and the UK were so significant that Alex Salmond, the Scottish first minister, would need a Plan B to use a new or different currency.

"The plan B is a fully separate ­currency," Cable said, signalling a hardening of the UK government's stance on the proposal. "The logic of what the governor and other people have spelled out is that the problems of a currency union with an independent Scotland are so difficult, so tricky, that it would almost certainly prove to be in Scotland's interests – and indeed the rest of the UK – that Scotland did have its own currency."

The Scottish National party said it would be "absurd" for the UK to damage its trading links with an independent Scotland by failing to maintain a monetary union.

A spokesman for John Swinney, the Scottish finance secretary, said: "The pound is as much Scotland's as it is the rest of the UK's, and the [Scottish government's] fiscal commission working group, with experts including two Nobel laureates, have concluded that it's in the interests of both Scotland and the UK to continue to retain sterling in a formal monetary union."

On Tuesday, Osborne again claimed that a currency union was highly unlikely, saying: "I don't think a workable currency can be created." BP's Dudley this week voiced "concern" over Scotland's monetary future.

Carney said last week that the euro crisis had shown that a sterling pact would require stringent and mutually agreed policies to control the fiscal policies of both governments – a catch-up category covering taxation levels, state debt and government borrowing.