Labour is facing a drop off in confidence among business leaders amid plans for tax rises and improvements to workers' rights, according to a survey.
The Institute of Directors (IoD) had noted a leap in optimism in July among its membership as the new government came to power.
But its latest economic confidence index showed a slump from a three-year high, falling into negative territory in August.
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Major indicators to show the biggest declines included business investment and employment.
Others to fall back were expectations for revenue, exports and wages.
Recent data has shown the UK economy to have the fastest economic growth in the G7 over the first half of the year.
Prime minister Sir Keir Starmer and his chancellor Rachel Reeves have made securing growth the "top priority" but complain that their plans are being complicated by a legacy £22bn black hole in the public finances.
"Tough choices" they have already announced, ahead of the 30 October budget, have included cutting winter fuel payments for all pensioners.
Critics argue that the tough choices include caving in to union demands to avert strikes, racking up a £9bn bill across public sector pay awards.
Commentators widely expect hikes to wealth taxes, such as capital gains tax, in the budget as it would chime with Sir Keir's warning last month that those with the broadest shoulders would face the greatest burden.
The IoD survey findings represent a major turnaround in opinion.
Ms Reeves secured a strong relationship with business in the run up to the election as firms ran out of patience with the Conservatives, long complaining of a lack of communication and strategy.
IoD chief economist Anna Leach said of its findings: "It's disappointing to see last month's welcome uptick in business leader confidence snuffed out over the summer.
"It is notable that the sharpest drops in our economic measures are in investment and headcount expectations, whilst other measures have moved to a lesser degree, albeit in a likewise negative direction.
"The newsflow in recent weeks on employment rights and Autumn tax rises has dented confidence in the environment for business in the UK.
"As we head into a busy Autumn, we are calling on the government to take time to get policy design right for the long-term, and deliver the stable tax and policy framework needed to drive business confidence and investment.
"Further clarity on the industrial strategy and the business tax roadmap, in conjunction with more progress in engaging with business on workers' rights, would be welcome."
The findings chime with warnings that the budget should not seek to rake in cash at the expense of the economy.
Former president of the CBI, the Cobra beer founder Lord Bilimoria, said that fears of tax increases would spark an exodus.
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He called on the government to concentrate on growth, calling any rise in capital gains tax "a short-sighted move".
"Investors are not going to come here if you keep putting up taxes," he told the Daily Mail.
"It will not bring in more money; in fact, money will fly from this country."
His comments were echoed by lastminute.com co-founder Brent Hoberman, who told the newspaper it "does not make sense to scare off business investment".
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