The British Chambers of Commerce (BCC) said economic growth was forecast to slow next year to just 1% - its weakest since the financial crisis a decade ago.
By the end of 2020 the UK economy will have experienced its second weakest decade of average economic growth on record, said the BCC.
Its head of economics, Suren Thiru, said: “Our forecast points to several years of anaemic activity in the UK economy unless decisive action is taken.
“An expansion in government spending is likely to give a nudge to the UK economy, whoever wins the General Election. However, a slowing global economy and unrelenting political uncertainty are expected to weaken business investment, trade and consumer spending, limiting the UK’s growth trajectory.
“The downside risks to the UK’s economic outlook remain concerningly high. Worsening global trends and uncertainty over the course of Brexit present real dangers for the UK economy.
“More uncertainty in the aftermath of the General Election and a persistent lack of clarity on the UK’s future trading relationships could also result in more muted growth.”
BCC director general Adam Marshall added: “The needs of business have been glossed over in the General Election campaign, and our latest forecast shows the danger of allowing this to continue.
“As soon as the smoke clears from the election battlefield, economic growth must be put front and centre again in Westminster. No incoming government can deliver its promises to the electorate without healthy and thriving business communities, or without a clear and detailed plan for the UK’s future relationship with the EU.
“As long as businesses are held back by Brexit uncertainty, high up-front costs, skills gaps and poor infrastructure, we can expect growth to be mediocre, at best.”
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