Frictionless trade after Brexit will be impossible, the EU's chief Brexit negotiator has said.

Michel Barnier made the comments to an audience at the Queen's University in Belfast just days before the UK is set to leave the EU.

"The UK has chosen to become a third country; to leave the Single Market and the Customs Union; to leave behind the EU's framework of common rules, common supervision and common Court of Justice," Mr Barnier said.

"It has chosen to create two regulatory spaces. This makes frictionless trade impossible. It makes checks indispensable."

Speaking to politicians, including former taoiseach Bertie Ahern, as well as business and community leaders, Mr Barnier confirmed checks will take place.

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"We will need sanitary and phytosanitary checks on food products and live animals," he said.

"The EU must be able to assess risks on any product coming into its market and, if necessary, activate physical controls," said Mr Barnier. 

"These checks must take place somewhere," he continued.

"And as the whole point of the protocol is to avoid a hard border and protect the all-island economy, it was clear that they could not take place at the land border between Ireland and Northern Ireland.

"The only real option was to use Northern Ireland's other entry points. This is also where such checks are the easiest to implement. And controls will also take place in Dublin and other EU entry points," he said.