Wages have fallen, economic growth is weaker and the UK's reliance on the European Union for trade has increased since the referendum to leave the EU, a new study reveals.
Research by the TUC showed that the economic position of the UK has weakened over the past year, with household finances being "squeezed".
The union organisation said its findings should spark the Government into taking action to strengthen the economy in preparation for Brexit, including investment in infrastructure and guaranteeing workers' rights.
The report, published ahead of the opening day of the TUC Congress in Brighton, said growth had slowed in 11 industries in the year to June, with only three employment sectors seeing real wage rises.
TUC general secretary Frances O'Grady said: "Our test for a successful Brexit is whether working people are better off. Throughout Brexit, the Government must protect jobs, and deliver stronger growth that reaches workers' pockets through higher pay.
"Successful Brexit negotiations are essential, but not enough. We have to deal with Britain's long-standing problems like low productivity and weak public investment.
"But with just 18 months to go before we leave, the economy is growing weaker, and Britain is simply not investing enough in the infrastructure we need to compete with the world's strongest economies.
"The Chancellor must take action in the Budget to boost public investment. This would help restore business confidence and get firms investing more too, and ministers must set out a modern industrial strategy that gives workers more say to help boost productivity."
A Government spokesman said: "As we leave the European Union, we will ensure that workers' rights are fully protected and maintained and we will build on our economic success by establishing a deep and special partnership with the EU while embracing the wider world as an independent, open, trading nation."
Shadow chancellor John McDonnell said: "This is further damning evidence that after seven years of Tory failure in the economy, it is working people paying the price.
"Real wages are falling with what economic growth there is coming to depend on eye-watering levels of household debt and fewer and fewer households able to save.
"The Tories are rushing into a cliff-edge Brexit with the economy sputtering.
"Only Labour can deliver the investment and industrial strategy we need to deliver good, secure jobs across the whole country, and we welcome the TUC's call for an immediate increase in infrastructure investment."