Mr Barnier said the UK now had two options, quit the EU without a deal, or seek an extension to Article 50.
Speaking in Brussels, Mr Barnier said: "If the UK Parliament does not vote in favour of the Withdrawal Agreement in the coming days only two options would remain.
"Leaving without an agreement or requesting a longer extension of the Article 50 period."
The EU’s chief negotiator said that if the UK did crash out on 12 April without a deal, the conditions set in the proposed Withdrawal Agreement would need to be met before any future talks to reestablish trade links - namely, the £39bn divorce bill, citizens rights and a solution for the Irish border.
Mr Barnier added: "The UK may ask for another extension. Such an extension would carry significant risks for the EU. Therefore a strong justification would be needed.
"We have always said that we can accept a customs union, or relationship along the style of the Norway model. The Political Declaration today can accommodate all of these options already.
"But if the UK so wishes we are ready to rework the Political Declaration."
Mr Barnier later told the European Parliament's Committee on Foreign Affairs that no-deal would disrupt EU/UK security co-operation.
"There will be a break in the level of talks, less mutual commitment, risks to intelligence pooling. There might be inconsistencies in applying sanctions regimes because of a low level of co-operation.
"The UK would no longer be taking part in EU operations or in the European Defence Agency's capacity-building programmes."
Mr Barnier said: "No-deal for some time poses the threat of there being no organised framework... Come what may, we must fend off the risk of strategic divergence."