British prime minister Theresa May’s government on Thursday repeated its opposition to negotiating a future UK-EU customs union, after the opposition Labour Party named that as its price for backing her Brexit plan. The rebuff came after an official in Brussels said EU president Donald Tusk had told Mrs May that the plan from main opposition Labour leader Jeremy Corbyn “might be a promising way out of the impasse”. Mr Corbyn wrote to Mrs May ahead of her meetings with EU leaders in Brussels on Thursday, setting out his terms for supporting her withdrawal deal following its rejection by MPs last month. He noted the EU has rebuffed her efforts to modify the controversial Irish ‘backstop’ arrangement in the deal, but outlined changes to an accompanying political declaration on future ties that he said would secure his backing. These include a commitment to a “permanent and comprehensive” UK-EU customs union, in which Britain has a “say” on future European Union trade deals. Mrs May has repeatedly said that a customs union would stop Britain signing its own trade deals, and her Downing Street office repeated this on Thursday. “It’s welcome that the leader of the opposition is engaging in this and it’s important that we continue to hold discussions to find a way forward to deliver Brexit,” an official said. “We are looking at those proposals with interest but there are obviously very considerable points of difference that exist between us. “The prime minister continues to believe that an independent trade policy is one of the key advantages of Brexit.” In his letter, Mr Corbyn also called for “close alignment” with the EU’s single market, underpinned by shared institutions and obligations. He said there should also be “dynamic alignment on rights and protections”, commitments to maintain Britain’s participation in EU agencies and funding programmes, and continued access to security databases and the European Arrest Warrant. “My colleagues and I look forward to discussing these proposals with you further, in the constructive manner in which they are intended, with the aim of securing a sensible agreement that can win the support of parliament and bring the country together,” Mr Corbyn wrote. Mrs May’s Brexit deal was rejected by the House of Commons last month, but a majority then voted to send her back to Brussels to renegotiate the backstop. This is an insurance plan that would keep Britain aligned to EU trade rules until another way was found to keep open the UK’s border with the Republic of Ireland. The Downing Street official noted that Mr Corbyn’s plan had caused a “fairly lively debate” within the Labour Party. “Seriously? Offering to help Tory govt enable Brexit?” tweeted Chris Leslie, one of several Labour MPs who reacted with fury at Corbyn’s letter. Another, Chuka Umunna, added: “This is not opposition, it is the facilitation of a deal which will make this country poorer.” Meanwhile, The Sun reported late on Thursday that Mrs May has approached a number of members of parliament from Labour to put forward an amendment to her withdrawal motion. Mrs May is planning to back a new package of workers' rights in a deal with some members of the Labour Party, the newspaper said, citing sources.