I wrote a few days ago that The government is fighting hard to ensure it does not have the right to cancel the Article 50 unilaterally. 

The UK Supreme Court has just ruled that the case should be heard by the ECJ as planned at the end of the month.

The government insists it will not withdraw Article 59. That is not the point at issue: they could decide not to withdraw it without needing the ECJ to rule they can't.