The Conservative leadership frontrunner Liz Truss has ruled out energy rationing this winter as she clashed over the cost of living crisis with her rival, Rishi Sunak. The foreign secretary rejected the proposal at the final Tory hustings, despite it being a key fall-back measure in the government’s “worst case” contingency planning. However, Sunak said “we shouldn’t rule anything out” after the French government warned it may have to ration energy, urging company bosses to take steps to curb consumption. As he made his final pitch to party members, the former chancellor added: “The challenges we face with this crisis are significant. Many European countries are looking at how we can all optimise our energy usage, that is a sensible thing for us to be doing as a country.” Asked by the LBC broadcaster Nick Ferrari at the last hustings before voting closes on Friday whether she could rule out energy rationing, Truss replied: “I do rule that out. Yes.” Under the government’s latest “reasonable worst case scenario”, published in August, businesses and even consumers could face blackouts this winter as concerns grow over power supplies. Officials believe that without energy rationing, the UK could experience blackouts for several days in January if cold weather combines with gas shortages to leave the country short of power. Gavin Barwell, Theresa May’s former chief of staff, suggested Truss was “crazy” to rule out energy rationing. He tweeted: “So if it is a cold winter and there simply isn’t enough energy to go round – which is a real risk – we are just going to have random blackouts rather than the government rationing non-domestic use so that vulnerable people don’t find themselves without heating.” Truss also ruled out a further windfall tax to pay for cost of living support for struggling families if she makes it to No 10, despite Treasury insiders believing that this may be necessary. She also pledged to Tory members at the Wembley arena event that she would introduce no new taxes if she became prime minister. In contrast, Sunak defended bringing in a windfall tax to pay for extra household support during his time as chancellor, adding: “It is absolutely the right thing to do when the energy companies are making billions of pounds of profits because of a war.” Truss told Tory members the new chancellor, expected to be the current business secretary, Kwasi Kwarteng, would look at more household support, but has provided little detail on what this might involve. She also admitted this would be her third priority, after cutting taxes and increasing energy supply. Pressed on support for businesses to cope with energy costs, she added: “You’ll have heard me talking about supply of energy and that’s why I think dealing with supply is the answer to this problem because you’re right, it’s not just a problem for people, it’s a problem for businesses with high energy costs. “So, I will be looking across the board to make sure we’re increasing supply and therefore dealing with the root cause of the issue rather than just putting a sticking plaster on, but I would absolutely be looking to act on business energy costs.” Asked whether she could repeat Boris Johnson’s 2019 pledge not to put up taxes – which he then broke – she replied: “Yes. No new taxes.” Truss denied that she had done “nothing” over the last five weeks while the cost of living crisis escalated after Ferrari suggested the Tory leadership race was a “five-week beauty contest” and she was a member of a “zombie government”. She committed to scrapping smart motorways where they have been found to be unsafe, while also hinting at making motorway speed limits advisory. Truss was asked whether she would keep Johnson’s extravagant gold wallpaper and expensive furniture in the Downing Street flat. She replied: “I’m from Yorkshire so I do believe in value for money and not buying new things if you’ve already got things that are perfectly good to use. I don’t think I’m going to have time to think about the wallpaper or the flooring.” In his last ditch bid to woo Tory members, and watched by his wife and parents, Sunak contrasted himself with Johnson, promising to reappoint an ethics adviser after two quit under the outgoing prime minister. He said: “I will lead a government that is conducted competently, that is conducted seriously, and with decency and integrity at the heart of everything that we do. That is the change that I am going to bring.” He promised his leadership would “start by being straight with the country about the economic challenges”, adding: “I’ve not chosen to say the things that people may want to hear, I’ve said the things I believe our country needs to hear. Although it hasn’t made my life easy, it is honest and, for me, that is what leadership is all about.” The two senior Tories made their final pitches to become next leader to about 6,000 Tory members at Wembley arena in London before voting closes at 5pm on Friday. Sunak won the MPs’ stage of the contest but entered the head-to-head part knowing that he was up against the darling of the Tory right. Over the last seven weeks the momentum has swung firmly behind Truss. The most recent polls suggest the foreign secretary has a lead of about 30 points among Tory members, but she faces a heavy in-tray with inflation topping 10%, and set to go higher, and energy bills soaring as the cost of living crisis grips the nation.