Ireland cannot accept a post-Brexit border backstop that has a time limit or a unilateral exit clause, Prime Minister Leo Varadkar said on Sunday.

"We are happy to discuss with the United Kingdom assurances that they may need to give them further confidence that the backstop were it ever applied will not be permanent," Varadkar told reporters.

"We are not playing chicken, we are not playing poker, we are just standing by our position which has been solid since day one," he said. "A backstop that includes a time limit isn't a backstop at all, nor would a unilateral exit clause work for us."