Ministers would have to consult the official watchdog on major tax and spending changes under Labour plans that would prevent a repeat of Liz Truss’s ill-fated mini-budget, the shadow chancellor, Rachel Reeves, has said. The move would ensure the Office for Budget Responsibility (OBR) was not “gagged” by future prime ministers and chancellors trying to avoid an official financial forecast, she said. A Labour government would also introduce legislation to ensure that the OBR had the power to independently publish its own impact assessment of any major fiscal event making permanent tax and spending changes, she pledged before a visit on Friday to the London Stock Exchange with the party leader, Keir Starmer. The plan was announced before the first anniversary of Truss and her chancellor Kwasi Kwarteng’s £45bn spree of unfunded tax cuts in a “mini-budget” that triggered a chain of economic chaos ranging from a sterling slump to soaring mortgage costs. In a break with tradition, which further spooked international markets, Kwarteng refused to publish the OBR autumn 2022 forecast. When it was published 10 months later after a freedom of information request, it revealed he went ahead with a package of tax cuts despite being told by the OBR that the economy was on course for a year-long recession and that higher interest rates were pushing up the cost of servicing the UK’s debts. “I’m saying that we won’t allow that to happen again because the OBR will not be gagged,” Reeves told BBC Radio 4’s Today programme on Friday. The party said ministers would be forced to open their books to the forecasters, though any government wanting to disregard them could seek to reverse the legislation. Reeves also defended plans to fix budget announcements for the autumn, followed by a spring update in early March, to give people and businesses time to prepare for changes. Rejecting the assertion that it would make it harder for chancellors to respond to unpredictable events, she said: “This is good international practice, that you set a date for the budget and you stick with it.” Reeves said the proposals would act as a “fiscal lock” to prevent the turmoil seen under Truss. “The economic damage done by the Conservatives’ mini-budget was nothing short of disastrous, and Britain is still paying the price, with higher mortgages, higher energy bills and higher prices in the shops,” she said. “Never again can a prime minister or chancellor be allowed to repeat the disastrous mistakes of last year’s mini-budget.” Boris Johnson also considered not going to the OBR before the first budget of his premiership, but this was rejected by his then-chancellor, Sajid Javid, according to a book by the Telegraph’s political editor, Ben Riley-Smith. Truss has recently been seeking to mount a defence of her positions in government, arguing earlier this week that the economic damage under her rule was largely the fault of “groupthink” from officials and the media, implying that the OBR was among those that backed a failed economic model. Asked why she did not have the OBR assess the mini-budget before it was delivered, Truss said it had not been asked to check much bigger spending commitments during Covid.