Professor Jo Shaw, from the University of Edinburgh, said that when new states were created citizenship was a personal choice and "always voluntary".
She said Scots who made such a choice would be considered ex-pat Brits and could continue to vote for a time in Westminster elections.
There were "no circumstances under which [it] could be or should be imposed on anyone", she said.
And she told members of the House of Commons Scottish Affairs Committee that although by law they would be Scottish citizens they would not have to identify as such and could refuse to vote or have a Scottish passport.
But her evidence was contested by another academic, who suggested problems would arise if citizenship was "entirely voluntary". Professor Bernard Ryan, an expert in migration law at the University of Kent, said ministers had to have a way to assess those who did not, for whatever reason, make a choice about their nationality.
He also agreed with suggestions from MPs that while many Scots might want dual nationality, the UK Government could refuse to grant it – although he added: "The history of Britain is one of acceptance of dual nationality."
MPs are investigating the possible ramifications of Scottish independence.
Asked if people could opt not to become Scottish citizens, Professor Shaw told the committee: "Citizenship is always a voluntary act on behalf of a person. There are no circumstances under which citizenship could be or should be imposed on anyone.
"If you look at British citizenship law at the moment, it has no provisions for depriving people of citizenship simply because they are not residing in the territory of the UK. One assumes that you could say the UK will not change its law. So at that point of independence those people would simply be British citizens who are not residing in the UK."
Over time, their rights and obligations would change, she told MPs, but "they would not lose their British citizenship and it would be entirely their choice whether they became Scottish citizens or not".
Ms Shaw said that as ex-pats they could be allowed to vote in Westminster elections for 15 years after Scotland left the UK.
This rule usually applies to those who move abroad and who vote in their former constituencies.
Speaking to The Herald on the issue of Scottish citizenship, Ms Shaw added: "They would be Scottish citizens by operation of law but they would not have to identify that way or to exercise the rights that came with that."
However, she said their children or grandchildren would not necessarily retain British citizenship.
The Scottish Government said its 2009 statement on the issue had not changed.
At that time, ministers said citizenship in an independent Scotland would be based on an inclusive model, saying: "Many people in Scotland have ties to the rest of the United Kingdom, including familial, social and economic connections. An independent Scotland could recognise the complex shared history of Scotland and the UK by offering shared or dual citizenship."
However, Ian Davidson, the chairman of the Scottish Affairs Committee, said the hearing illustrated the complexities surrounding citizenship, passports and immigration in an independent Scotland. He said: "The Scottish Government has got to come clean on these issues."
l Jim McColl, one of the country's richest men and chief executive of engineering firm Clyde Blowers, has declared his support for independence after previously backing calls for a strengthened form of devolution instead.
He said: "It appears that only independence - will allow England and Scotland to pursue distinct economic policies in the face of different demands and competitive pressures."
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