Lord Lang said the upcoming Great Repeal Bill would be “an extremely complicated piece of legislation” and warned the Scottish Parliament would not necessarily get a say on all the powers set to be handed over by Brussels in devolved areas.
The comments come amid a growing row over the future of devolution after Brexit, with Theresa May suggesting new powers over farming and fishing could be retained by Westminster.
The Great Repeal Bill will scrap the European Communities Act, ending the authority of EU courts and transposing EU regulations into domestic law, allowing them to be altered or removed after Brexit.
A report by the House of Lords Constitution Committee, which Lord Lang chairs, said ministers should strike a deal with devolved administrations on which new powers they can expect as a result of the process.
The report warns that the volume of regulations means not all will be dealt with through primary legislation, denying MPs a vote. In that scenario, MSPs may not be asked to consent if EU regulations affecting devolved responsibilities are changed.
The report warns the process will put “considerable strain” on the constitution unless there is “consultation and consent” between the UK and devolved governments.
“It’s inconceivable the bulk of it can be done in primary legislation,” Lord Lang said. “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 report from peers calls on the Prime Minister not to “pick and choose” which elements of European law she wanted to scrap or alter without parliament’s full involvement. Peers were warned yesterday they could “incentivise” the European Union to offer Britain a bad Brexit deal if they pass a further amendment to the Article 50 Bill.
The House of Lords is expected to vote on Tuesday on an amendment calling for Westminster to be given a “meaningful” vote on the withdrawal agreement secured by the Prime Minister during negotiations under Article 50 of the EU treaties.
Mrs May has promised parliament a vote, but only on a “take it or leave it” basis, which would see the UK crash out of the EU without a deal if MPs reject the agreement she obtains.
Many peers are insisting that they should be given the option of telling ministers to go back to the EU and renegotiating a better deal.
Mrs May’s official spokesman said: “If we are in a position where any deal negotiated by the Prime Minister could be rejected by MPs, that gives strength potentially to other parties in the negotiation.”
Former Tory chancellor Lord Lamont warned peers to “see sense” and claimed Brexit is “under attack” on several fronts. In a speech in central London, he warned adding conditions to the Bill is “not the same as scrutiny”.