Peter Byrne - WPA Pool/Getty Images The UK will leave the EU's trading rules in less than five weeks' time. With or without a trade deal with Brussels, Brexit will inflict a significant hit on the UK economy, the Office for Budget Responsibility said this week. No deal would reduce long-term output by 6%, it said. However, the cost and disruption that awaits the UK didn't feature in Rishi Sunak's Spending Review. The Chancellor and the rest of Boris Johnson's government are reluctant to talk about Brexit due to its associated costs, Theresa May's former Europe adviser told Business Insider. Experts agreed that the government's refusal to be upfront about Brexit was hindering preparation. Visit Business Insider's homepage for more stories. The UK will break away from the European Union's trading rules in less than five weeks, and significant disruption and economic pain awaits.