Leaving the EU without a deal on 29 March is not the “legal default”, as has been repeatedly, but wrongly, asserted. The UK can only leave the EU in accordance with domestic constitutional requirements. The Supreme Court, the highest judicial authority responsible for interpreting our unwritten constitution, ruled in Miller that Brexit requires an Act of Parliament. It would be contrary to the principle of parliamentary sovereignty to interpret the legislation merely permitting the Government to trigger the Article 50 process as amounting to authorisation of withdrawal on whatever terms were negotiated or, failing that, without any deal at all. Accordingly, the Article 50 notice was conditional on a further Act of Parliament.
As such, it would be unconstitutional, and therefore unlawful, for the UK to leave the EU without a deal on 29 March in the absence of legislation to such effect. The EU cannot force a Member State out of the Union against its constitutional traditions; to do so would be directly at odds with ECJ jurisprudence, the EU Treaties as a whole and Article 50 in particular. However, the EU is not obliged to offer an extension of time. If refused, in the absence of an Act of Parliament authorising Brexit in one form or another, the conditional Article 50 notice will simply lapse on 29 March, if not unilaterally revoked. This is the legal default, not an automatic no-deal Brexit.
For further reading please click below