WESTMINSTER must pay for any checkpoints at Scottish ports if Northern Ireland is kept in the customs union after Brexit, campaigners have said.

Officials from the Britain and EU want to effectively keep Northern Ireland in the trading bloc when the rest of the UK leaves, in order to avoid a hard border across the island of Ireland.

However, Green MSP Ross Greer warned that ports such as Cairnryan, and potentially Ardrossan, would become customs borders "overnight".

Greer said that the UK Government must pick up the tab for any infrastructure costs that would create.

He said: "If the Tories insist on this ridiculous policy of leaving the Customs Union, then it’s likely that Northern Ireland will remain in to avoid the return of a hard border on the island of Ireland.

"The consequences of that for Scotland, and our west coast in particular, would be massive. The likes of Cairnryan and potentially Ardrossan and other ports would become customs borders overnight. That would require enormous amounts of additional infrastructure, both physical and digital.

"Given that not one of the councils along our west coast, or Scotland as a whole, voted for Brexit or to leave the Customs Union, neither they nor the Scottish Government should be forced to fork out what will be significant sums of money in a short space of time to make these changes. Every penny should come from the Westminster government dragging us into this mess."

Greer said that Brexit secretary David Davis and Scottish Secretary David Mundell should commit to paying for any Brexit port costs.

In response, a UK government spokeswoman said: "The whole of the UK will be leaving the EU’s Customs Union.

"By leaving the Customs Union and establishing a new and ambitious customs arrangement with the EU, we will be able to forge new trade relationships with our partners around the world, and maintain as frictionless trade as possible in goods between the UK and EU.

"The UK is committed to avoiding a hard border between Northern Ireland and Ireland, including any physical infrastructure or related checks and controls."