Michel Barnier has warned of the risk of a hard border returning in Ireland.
The EU's chief Brexit negotiator urged rapid movement on the vexed issue ahead of this June's meeting of the bloc's leaders.
He also said there could be no withdrawal deal without a "backstop" option, meaning if no better solution is found Northern Ireland would continue to follow EU rules relating to the all-Ireland economy and North-South co-operation.
Mr Barnier said: "The backstop is not there to change the UK's red lines. It is there because of the UK's red lines.
"The UK's decision to leave the single market and the customs union creates a risk that the hard border will return. This is why it is necessary to have a self-standing backstop solution."
Many operational details have yet to be resolved surrounding the UK's only land border with an EU state after Brexit and the issue is top of the agenda in Brussels.
Mr Barnier visited the Irish border town of Dundalk on Monday.
He said: "We need to agree rapidly by June on the scope of all-island customs and regulations, the safety and controls that we need to respect the single market."
This summer's meeting of European leaders in Brussels would be a "stepping stone" for the final summit in October, which is the deadline for reaching an agreement on withdrawal, he added.
"The backstop is needed in order to respect the integrity of the single market and the EU's customs union.
"Some people think that we could have two different sets of rules on the island of Ireland and still avoid border checks.
"But Ireland is a member of the EU - and a proud member, I add. It is an active player, active, very active player, in the single market.
"Goods that enter Ireland also enter the single market. It is called the "single" market for a reason.
"So, since we all agree that we do not want a border, and since the UK agreed to respect Ireland's place in the single market, then that means goods entering Northern Ireland must comply with the rules of the single market and the Union Customs Code."
A joint report on the UK's withdrawal agreed in December by Prime Minister Theresa May and European Commission president Jean-Claude Juncker included both British proposals, along with the third "backstop" option which would keep Northern Ireland in the customs union.
But a version published by the EU in February and agreed by the EU27 in March contained only the "backstop", effectively drawing a customs border down the Irish Sea, which a furious Theresa May said "no British prime minister could ever agree".
This week marks Mr Barnier's third visit to Ireland and Irish foreign affairs minister Simon Coveney said he was a friend of the country.
After delivering the keynote speech during a meeting of the Irish government-hosted All-Island Civic Dialogue on Brexit in Dundalk, Mr Barnier crosses the frontier to Newry in Co Down for meetings with business leaders.
On Tuesday he will visit the other, north-western end of the porous 310-mile border at Londonderry.
DUP leader Arlene Foster has said he did not understand unionist culture.