THE UK Government’s complex task to transpose all EU law onto the UK’s statute book must be carried out in a way that safeguards the “coherence of the Union,” peers insist today.

The House of Lords Constitution Committee warns Theresa May not to use the forthcoming Great Repeal Bill to avoid full parliamentary scrutiny.

The legislation will scrap the European Communities Act, ending the legal authority of EU law in the UK. It will also transpose EU regulations into domestic law, crucially allowing them to be altered or removed once withdrawal has taken place.

Lord Lang, the Committee Chairman, said it was important to get the safeguards right, particularly given the Brexit campaign’s promise to "take back control".

The former Conservative Scottish Secretary explained: “It’s inconceivable the bulk of it can be done in primary legislation. Where there are major policy changes, for example, on immigration or customs law, primary legislation would be needed.

“For the rest, there are something like 20,000 bits of EU legislation, which we will have to bring back to the UK in one form or another and we won’t know precisely what they are finally until the negotiations are complete. So there is a huge bottle-neck looming; that’s what we are trying to prepare for.”

The Scottish peer accepted most of the bill’s provisions would have to be enacted through secondary legislation. While David Mundell, the Scottish Secretary, has indicated that he “anticipates” Holyrood would have to pass legislative consent motions to enable Westminster to legislate on devolved matters, secondary legislation is not subject to the so-called Sewel Convention.

The report notes how there are two ways forward with regard to the bill and Scotland; either it will enable the UK Government to change the body of EU law in preparation for Brexit with Holyrood taking responsibility for those matters within its devolved competence afterwards or the Scottish Government can in the first instance propose amendments to the legislation on devolved matters.

“If the former, then the devolved institutions will need to be appropriately consulted on the amendments to EU law in areas that fall within their jurisdiction. If the latter, it is essential that the devolved institutions work closely with the UK Government to ensure that EU law does not ‘fall between the cracks’ of their respective jurisdictions and that decisions on the repeal or adoption of domesticated EU law are taken in a way that has regard to the coherence of the Union.”

Lord Lang made clear he was optimistic that with goodwill on all sides as well as proper scrutiny and consultation with interested parties that the complicated process could be successfully completed.

The former Cabinet minister stressed how the purpose of the UK Government was to make the legal transition as fair, smooth and sensible as possible and that it would be others, who would “always look for excuses to stir up a sense of grievance”.

Asked if he believed the SNP leadership was pushing matters towards a constitutional crisis, Lord Lang added: “It will be incomprehensible for anybody to force some sort of crisis now before negotiations have even started and to claim that Brexit is the reason for them. That would be preposterous and would be seen to be preposterous by the vast majority of the Scottish people.”

Stephen Gethins, the SNP’s Europe spokesman, said: “Devolved administrations must be properly and officially formally involved when it comes to matters that fall within their competence.

“The increasing danger, however, is not powers for devolved administrations ‘falling through the cracks’, rather the risk of a Westminster power-grab. Powers must come back to Holyrood and not stop in a centralised Westminster."