A British decision to leave the EU would significantly change the border between the Republic of Ireland and Northern Ireland, a top economist has said.

Britain leaving the European Union could transform the Irish border into a major international frontier manned by passport checkpoints and customs control, it has been warned.

Influential economist Colm McCarthy told an Irish parliamentary watchdog the country must react swiftly in the event of a Conservative victory in the UK elections in May over a promised in/out referendum.

Talks would begin between London and Brussels far in advance of any "Brexit" ballot, and Dublin would need a key role at the negotiating table to bolster the country against trade, immigration and financial fall out, he cautioned.

"If the Tories don't win the election, then problem solved and the whole thing may never arise," he told TDs and senators in Dublin's cross-party EU Affairs committee.

"But if they do I think we need to get involved quickly. It seems to me that creating a fresh, serious frontier between the north and south of Ireland, that is an issue that is quite unique to us.

"I think we are quite entitled to draw that to people's attention and make it clear that this is a big issue for us, and that we need to get involved early."

Mr McCarthy, a former economist with Ireland's Central Bank and the Economic and Social Research Institute think-tank, has been commissioned by the Irish government to author several influential economic studies.

Asked to appear before a special hearing into the ramifications of Britain leaving the EU, he said the Irish border - the only land border Britain has - would be of critical importance.

"If the British leave and the border becomes the external frontier for the European Union, that could be very awkward for the border counties," he said.

"You could end up with passport controls, customs inspections and all the rest of it at that frontier."

Comparing it to Croatia, which joined the EU last year, he added: "You've suddenly got a lot more people marching around in uniforms and peaked hats and making life awkward."

Ireland has never been in the EU without its closest neighbour, they both joined on the same day in 1973.

Mr McCarthy said exit would probably mean Britain - Ireland's biggest trading partner - seeking closer trade ties with the US and possibly Canada, with which it has historical links.

This would leave Ireland as the only "Atlantic country" in the European bloc, which he said could have some benefits to Dublin but there would also be many threats.

Main areas of concern would be freedom of movement for people, trading freedom and agricultural agreement, Mr McCarthy said.

The University College Dublin economist added that Ireland leaving the EU along with Britain should not be totally ruled out as an option.

"The Irish and UK economies are more integrated than Ireland is with the EU," he said.

Fears already abound for London's status as Europe's financial services capital, which could be threatened by Paris or Frankfurt if the EU were to use oversight bodies "to screw the competitiveness of the City of London", he added.

This would hit Dublin's International Financial Services Centre (IFSC), which acts partly as a satellite to the City, he warned.

Mr McCarthy said all the opinion polls were pointing to a hung parliament in Westminster later this year, and there was no certainty that the British people would vote for Brexit if it went to referendum.

However, if Prime Minister David Cameron were to secure a majority, he said Dublin would need to be poised to get quickly and closely involved with any pre-referendum talks, which would either hammer out concessions or agree the shape of a "divorce settlement", he said.

"We have a very legitimate reason for getting involved," he said. "This matters more to us than any other country in Europe."