Britain could remain closely aligned with the rules of the European Union's single market under its initial withdrawal deal with Brussels, a move that has critics worried about heading towards a soft Brexit.

The deal reached Friday after nearly six months of negotiations detailed the key Brexit issues -- Britain's divorce bill, EU citizens' rights and the sensitive Northern Ireland border.

If European leaders meeting next week sign off as expected on the 15-page document, which is set to form the basis of any final withdrawal agreement, negotiations can move on to the post-Brexit relationship.

However, the deal's commitment to maintaining an open Irish border has sparked debate in Britain over whether it offers an unexpected backdoor to closer ties with the European single market than previously envisaged.