... a transition cannot be delivered by the European Union (Withdrawal) Act, which was passed just over a month ago. That Act operates on the assumption that the UK will leave the EU on exit day. It chooses that day as the `switch-over' point, after which the European Communities Act 1972 (ECA) is repealed and most (but not all) EU law is converted into domestic law (or 'retained EU law').

If exit day is to stay at 29 March 2019, but the UK is to give effect to EU law `proper' (rather than `retained' EU law) beyond that day, fresh legislation is needed. A new Act of Parliament will have to amend the Withdrawal Act before the new scheme of `retained EU law' replaces the ECA.

and

The EU Treaties would cease to apply to the UK two years after the notification of an intention to withdraw [i.e. March 29, 2019] and the ECA is repealed at the same time. There can be no `transition period' without a withdrawal agreement.

Emphasis added

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre