BELFAST, Britain, April 9 (Xinhua) -- The 500-km-long border that separates the Republic of Ireland from Britain's Northern Ireland is embroiled in a tug-of-war tussle at the epicenter of Brexit.

Some politicians and political commentators fear the fate of the border poses a threat to a divorce deal as Britain negotiates its exit from the European Union (EU).

Since the 1920s a Common Travel Area has more or less allowed people from either side of the meandering frontier to cross without going through any border controls. This freedom of movement arrangement was introduced as part of the creation in 1922 of the Irish Free State, later to become the Republic of Ireland.

The 32 counties on the island of Ireland were divided, in the north east of the island forming Northern Ireland, the others becoming part of the free state.

The Irish Republican Army (IRA) waged a campaign for re-unification of the island, leading to 30 years of troubles. During that period over 3,600 people were killed and thousands more injured. British troops patrolled Northern Ireland and communities were divided.

At the height of 30 years of troubles over Irish unification, many of the 200 recognized border crossings were closed and watchtowers built near major border points, manned by armed British troops.

The bombings, killings and assassinations came to an end 20 years ago when an agreement was signed to restore peace to Northern Ireland.

The watchtowers came down and borders reopened, creating an invisible border between the two parts of the island of Ireland. The border has thrown up a number of quirky situations, such as one house with some rooms in Ireland and others in Northern Ireland. Motorists heading along the N54/A3 major route cross the border four times as they navigate the twisting and turning highway.

As EU members, Ireland and Britain are part of the European single market and customs union, enjoying two decades of a border more invisible than at any time in history. British and Irish citizens have enjoyed access to each other's country without the need for passports as a result of laws dating back to the 1920s.

The frontier runs from Lough Foyle in the north of Ireland to Carlingford Lough in the northeast, separating the Republic of Ireland from Northern Ireland.

Every month around 177,000 freight trucks, 208,000 vans, and 1,850,000 cars cross the border, while around 30,000 people cross the border daily to travel to their jobs.

Some estimate that when smaller crossings are added, there are up to 300 roads, lanes and tracks linking the two parts of the island.

Politicians in Belfast, Dublin and London all want the border to remain frictionless when Britain quits the EU. An added concern is the impact border controls could have on the Good Friday agreement which ended decades of troubles.

The problem for Brussels is that once Britain's membership of the EU comes to an end that 500 km meandering line will become a land border between the EU and the UK.

Ireland will remain a member of the single market and customs union, with all the rules that apply such as free movement of all peoples from EU member states.

Britain wants to control its border and control immigration, but at the same time retain access to the single market.

In the latest round of negotiations between the EU and Britain, three possible scenarios were put forward by Brussels to resolve the border issue.

The EU's chief Brexit negotiator Michel Barnier published a draft withdrawal agreement which would see Northern Ireland effectively staying in the single market and customs union as a common regulatory area after Brexit as a way or avoiding a hard border. It would see Northern Ireland complying with all EU rules, leaving England, Scotland and Wales outside the EU.

The leader of the Democratic Unionist Party (DUP) Arlene Foster, whose party is with the most seats in the Northern Ireland Assembly and agrees to support a Conservative minority in Westminster, has rejected the proposal, describing it was "utterly unacceptable.

The Guardian commented that Barnier's solutions to the Irish border has pushed Brexit negotiations into a dangerous new phase.

The British government wants to see a frictionless border controlled by technological solutions, but the clock is ticking as all sides grabble with finding a solution to the border issue that some say poses a threat to Brexit.