If the government were to decide an extension of the transition period is necessary, what would need to be done to deliver it under UK law? Would another Act of Parliament be required? Not necessarily.
Delegated legislation may be enough, or maybe it would come down to the way the minister asks the question (and indeed, which minister is doing the asking).
According to Article 126 of the Withdrawal Agreement (WA) with the EU, the transition (or implementation) period is due to expire on 31 December 2020. However, Article 132(1) of the Withdrawal Agreement says that:
Notwithstanding Article 126, the Joint Committee may, before 1 July 2020, adopt a single decision extending the transition period for up to one or two years.
Boris Johnson has pledged not to extend the transition period. This was enshrined in law: the EU (Withdrawal Agreement) Act 2020 amends the EU (Withdrawal) Act 2018 to say (in s. 15A) that a ‘Minister of the Crown may not agree in the Joint Committee to an extension of the implementation period’.
This, at first sight, looks like a cast iron guarantee that the transition period will not be extended, unless an Act of Parliament is passed to amend s.15A.
However, section 41 of the EU (WA) Act 2020 says:
(1) A Minister of the Crown may by regulations make such provision as the Minister considers appropriate in consequence of this Act.
(2) The power to make regulations under subsection (1) may (among other things) be exercised by modifying any provision made by or under an enactment.
(3) In subsection (2) ‘enactment’ does not include primary legislation passed or made after the implementation period (IP) completion day.
According to schedule 4, paragraph 6 of the Act, ‘[a] statutory instrument containing regulations under section 41(1) is subject to annulment in pursuance of a resolution of either House of Parliament’. In other words, the regulation is enacted by negative resolution, and the approval of either House is not required.
So what does this mean? Section 41 is a so-called Henry VIII provision, giving ministers considerable powers, including to amend by secondary legislation ‘any provision made by or under an enactment’ (that is, an Act of Parliament), provided that Act is not ‘primary legislation passed or made after IP completion day’.
In other words, s.41 cannot be used to amend an Act of Parliament made after the end of transition, but it can amend an Act made before that day.
So it looks like the government could enact a regulation to amend the provisions of EU (WA) Act 2020 so that a minister could now agree to an extension.
Some might argue that this would be unconstitutional: any regulation attempting this would be beyond the scope of s.41, and the Henry VIII power should be read narrowly. Also, reading the legislation in this manner undermines the purposes of the Act.
We disagree. First, s.41 does not specifically preclude the clause being used to amend the EU (WA) Act 2020. It says that the minister may by regulations make ‘such provision as the Minister considers appropriate’.
Apart from the question of appropriateness (and that the Act must predate 31 December 2020), there is no other restriction on the extent of the power.
Second, just because Henry VIII powers are broad and give the executive considerable powers, does not mean they cannot be used. The Supreme Court, in the Public Law Project case  UKSC 39 did say that broad powers, especially those including a Henry VIII clause, will be construed narrowly.
But when it comes to the law, context is everything. In the Christchurch Borough Council case  EWHC 2126 (Admin), for example, the Henry VIII clause did not expressly include an ability for measures to have retrospective effect (changing things already done in the past).
So, surely that meant that it could not be used to do precisely that, given the need to read Henry VIII clauses narrowly? The Court disagreed. The retroactive effect here did not give rise to any unfairness. So the measures enacted under the Henry VIII clause were valid.
What, then, if regulations to amend s.15A of the EU (Withdrawal) Act 2018 were backed by a general consensus in Parliament to recognise the need for more time, as Covid-19 had meant that the UK had not had the time to negotiate the good future trade deal it desired? We would argue that s.41 could be used.
Which brings us to our third point. Would the use of the Henry VIII clause to repeal s.15A frustrate the purpose of the EU (WA) 2020, breaching Padfield ( AC 997)? Section 15A prohibits a ‘Minister of the Crown’ from agreeing in the Joint Committee to an extension of the implementation period.
But what if the Prime Minister asks? Does that have the effect of falling outside s.15A? The minister is not agreeing to an extension – he is proposing that the Joint Committee agrees to an extension initiated by the UK.
There still remains the problem of the ‘implementation period completion date’, currently scheduled for 31 December 2020 at 11.00 pm (s.39(1) EU (WA) Act 2020). This would need to be changed.
However, s.39(4) of the EU (WA) Act 2020 empowers a minister to ‘amend the definition of “IP completion day”’, again by secondary legislation adopted by negative resolution (Schedule 4, para. 5).
Why would s.39(4) give the minister the power to do something (change the day and time of the implementation period) if the rest of the legislation (under s.15A) prohibits him from doing that?
And how can it be the sole purpose of the Act for the implementation period completion date to fall on 31 December 2020 when the Act provides the government with the power to amend that date?
Clearly, having an Act of Parliament repealing s.15A and amending IP completion day would be the cleanest and neatest way of extending transition.
But no one anticipated coronavirus in autumn 2018 when the Withdrawal Agreement was finalised. And no one anticipated that coronavirus would not only hinder negotiations, but also prevent Parliament from sitting in the usual way.
By Catherine Barnard, senior fellow at the UK in a Changing Europe and Professor of EU Law and Employment Law at the University of Cambridge and Alison Young, Professor of Public Law at the University of Cambridge.