That argument is set to dominate politics for weeks to come as the government prepares for a Lords amendment to be voted on in the Commons.This is a dangerous issue for Theresa May. Her party has repeatedly torn itself apart over Europe and even now, as preparations for Brexit gather pace, it could topple her. The customs union has come to define the Brexit negotiations and the reason for that is the Irish border. If we leave the CU, as Mrs May has promised we will, it is difficult to see how a hard border can be avoided in Ireland. Two decades on from the Good Friday Agreement very few people would want to see anything but a frictionless border. Staying in the CU solves this problem. But it throws up others. If the customs arrangements remain as they are currently those shiny, new trade deals the country was promised after Brexit will be scuppered. Which begs the question what is the point of leaving the EU at all? And for Mrs May there are internal problems as well. The European Research Group of Tory MPs cite remaining in the CU as a Brexit red line and if it is crossed they have the numbers to spark a leadership challenge. So the actual vote when it comes next month has become crucial for Mrs May. If she loses it and parliament demands preparations are made to stay in the CU the prime minister faces the very real threat of a backbench rebellion which could see her fighting for her political life. North Norfolk Liberal Democrat Norman Lamb was a signatory to the debate. He said: I accept the outcome of the referendum. Now we need to strike a deal that secures our economic interests and that does not undermine our productivity. No one voted to put the Northern Ireland peace deal at risk, no one voted to erect barriers to trade that puts jobs at risk and for me there is an extremely powerful case for remaining in the customs union. I have no problem though, in the longer term, of negotiating a bespoke customs deal between the UK and the EU. But the vast majority of Conservative MPs will when the meaningful vote happens back the government and vote to quit the union. Sir Henry Bellingham, North West Norfolk, said: We have to come out of the customs union to have the flexibility to sign trade deals with other countries this is one of the great prizes of leaving the EU. We will go on trading with Europe of course we will. But the big increases in our trade with be with the Commonwealth countries and emerging nations like Brazil that is where the huge potential lies. And South Norfolk MP Richard Bacon added: After the UK leaves the EU, we need to be able to decide for ourselves what trade arrangements we make with other countries. We would not be able to do this if we stayed within the customs union. The customs union is part of the core structure of the EU. We are leaving the EU so, of course, we will also be leaving the EUs customs union. But Broadland MP Keith Simpson is confident a solution can be found. It seems to me that at least in the short term we might have to make some compromises over these final bits of negotiations not least because it is not yet clear how we resolve the issue of the Irish border, he said. That has got to be resolved. My view is that I want an arrangement when we leave that is not going to damage, political or economically, the UK. I am sceptical when people say when we leave it is going to be a disaster, but equally sceptical about those who say it is going to be a land of milk and honey. Ultimately we are crawling forwards to a compromise of some sort or another. As a Conservative MP I want to see a Conservative government finish the job. But there is a lot of theatre going on. This is a horrendously complicated process and I dont think it is helpful for me to lay down red lines and say if those red lines are breached I am going to vote against the government. Whether it is the ardent Brexiteers who are sending their letters in to the prime minister and threatening to start a leadership contest or the Remainers I dont think anyone wants to bring down the government. For Mrs May the customs union debate could still go either way.