STRASBOURG (Reuters) - The European Parliament’s negotiator on Britain’s divorce from the European Union warned on Tuesday against the risks of a return to violence in Northern Ireland because of Brexit.

“The worst thing that can happen is that we return to violence because of this,” Guy Verhofstadt told a news conference in the European Parliament in Strasbourg.

The former Belgian prime minister said a new hard border between the Republic of Ireland, which is staying in the EU, and the British province of Northern Ireland would be “politically a big problem and economically a disaster.”

Britain and Ireland, which broke away from British rule in 1922, both joined the EU in 1973, ensuring that the openness of their border remained, even during years of violence in the province involving groups favoring rule from either London or Dublin.

However, Brexit will transform the line across the island into the only land frontier between the European Union and Britain, raising the possibility of customs and other controls.

Verhofstadt said he had met representatives of Northern Ireland and cautioned EU and British negotiators to tread carefully on the issue.

The EU chief negotiator on Brexit, Michel Barnier, said last week he would do all he could to avoid upsetting the peace in Northern Ireland.