By Padraic Halpin and Amanda Ferguson
DUBLIN/BELFAST (Reuters) - Britain's plan for leaving the European Union is unlikely to be accomplished without significant changes to border arrangements between Northern Ireland and the Irish Republic, Sinn Fein President Gerry Adams said on Tuesday.
Northern Ireland, a British province, will represent the only land frontier between Britain and the EU once it leaves and leaders in London, Dublin and Belfast say they want to avoid the raising of a "hard" border, with customs checks and border posts.
Setting out her vision for Brexit on Tuesday, Prime Minister Theresa May said Britain will leave the EU's single market upon its exit and cannot remain a full member of the customs union, but will seek tariff-free trade with the bloc.
She said Britain will seek a practical solution to preserve a common travel area between it and Ireland predating their entry into the EU in 1972, while simultaneously protecting the integrity of the United Kingdom's immigration system.
"It is difficult to see how this can be accomplished without significant changes to the current border arrangements," Adams, whose party has shared power in the north for the past decade and is a growing force in the south, said in a statement.
Sinn Fein, which wants a referendum to unite the two sides of the border after 56 percent of Northern Irish voters backed staying in the EU, reiterated that the province should be given a special designated status within the EU once Britain leaves.
The pro-British Democratic Unionist Party (DUP), Sinn Fein's rivals and partners in Northern Ireland's devolved government before its collapse this week, said it was committed to helping May build a "prosperous and strong UK outside of the EU".
Irish Prime Minister Enda Kenny, who has called Brexit the biggest challenge Ireland has faced in 50 years, said he welcomed his British counterpart's willingness to look at the most effective and practical outcome to the border situation.
However one of his senior ministers called for more clarity from London.
"What is now of importance for us is we receive further clarity in relation to the status of the customs union and the United Kingdom's relationship with that," Public Expenditure Minister Paschal Donohoe told national broadcaster RTE.
(Editing by Angus MacSwan)