Scottish independence would create extra costs for BP, according to the energy giant's chief executive.
Bob Dudley said there are "quite big uncertainties" over currency, European links and tax regimes if Scotland becomes independent.
In an interview with the BBC, Mr Dudley said: "I'm not concerned but there's enough uncertainty and talk about it and questions raised.
"It would create extra costs for our business. We have to duplicate the centres and do things, and again the currency question I don't know the answer to."
BP's future in the North Sea in the event of independence "would depend on what it really led to", he said.
"These investments are big, they are under way, we want to see them developed. It depends on what tax regimes are there and it depends on currency."
He added: "We have got a lot of people in Scotland, we have got a lot of investments in Scotland.
"There's much debate about what would happen with the currency, and of course whether there are connections with Europe or not.
"These are quite big uncertainties for us, and at the moment we are continuing to invest at a pace because these projects are under way.
"But it's a question mark. I think all businesses have a concern.
"My personal view is Great Britain is great and it ought to stay together."
A Yes Scotland spokesman said: "A shared currency is in the overwhelming economic interests of both Scotland and the rest of the UK.
"This was the view of the (Scottish Government's) Fiscal Commission Working Group last year.
"With independence, the continued use of sterling has the overwhelming support of the people of Scotland and the public in the rest of the UK."
Better Together chief Alistair Darling said: "This is perhaps the biggest intervention by a major business so far in the referendum debate.
"I hope that more companies and business leaders speak out over the coming weeks and months. This debate is far too important to be left to politicians alone.
"Bob Dudley is quite right to express concern about the issue of currency. It is far from certain what currency we would use if we vote to leave the UK."
Labour's shadow energy minister Tom Greatrex said: "The warning of uncertainty, instability and impact on activity from BP today is one that echoes concerns expressed by others in the energy sector, including SSE, Scottish Power and Infinis.
"As Sir Ian Wood's influential interim report on the future of the North Sea made it clear that we need greater collaboration and co-operation to get the most from this diminishing resource. In oil and gas, as in other sectors, it makes no sense for us to be talking about putting up barriers and making business more difficult."
A Scottish Government spokesman said: "We welcome all contributions to the debate on Scotland's future, and would be happy to meet with Mr Dudley to discuss the future of the industry in an independent Scotland.
"He has made clear these are his personal views and that the company is continuing to invest in Scotland.
"Record amounts of money are being invested in the offshore sector, by BP and others, with £100 billion of investment planned by companies. And more than half of oil and gas reserves by value are still to be extracted.
"BP is a company which already operates in more than 80 independent countries around the globe, and an independent Scotland with full control of its economy and huge resources will offer an attractive and stable environment for businesses in the offshore and other sectors.
"And an independent Scotland will keep the pound as part of a sterling area, which is in the overwhelming economic interests of the rest of the UK."
Asked about Mr Dudley's comments, Prime Minister David Cameron's official spokesman said: "I think it is important when one of the UK's leading businesses and investors and an important employer raises this issue.
"He is making a very important point."
Several Government ministers have already made clear that it is "highly unlikely" that an independent Scotland would be able to join a currency union with the remaining UK, said the spokesman.
Scottish Secretary Alistair Carmichael said: " It is little surprise that global corporates like BP and SMEs around Scotland have concerns about the uncertainties of independence.
"They need to know what will happen with the UK pound, barriers to trade and the terms of EU membership if Scotland decides to leave the UK.
"The Scottish Government's White Paper has done nothing to clarify what the likely impact of independence would be on business and jobs. We have more and more unanswered questions and business must keep speaking up and demand clarity on what independence would mean for them."