The government should rethink its Brexit strategy, following last week's election, according to the engineering industry organisation, the EEF.
It said without a more pro-business stance, the resulting political instability may force more firms to alter their plans "away from the UK".
The EEF is the latest business organisation to call for a rethink of the government's Brexit plans.
It wants access to the single market to be at the heart of Brexit negotiations.
The EEF said even before the election firms were already altering or thinking about changing their business plans because of the Brexit vote.
Terry Scuoler, EEF chief executive, said the government had already "wasted a year" and needed to "move away from its previous rhetoric and start repairing relations with EU partners".
For the EEF that meant putting access to the single market and staying in a customs union at the centre of the government's negotiations and involving business groups in the talks over trade.
It is also calling for a "suitable" transition period to be "firmly back on the table" as part of the Brexit talks.
On Monday Carolyn Fairbairn, director-general of the CBI, called for the government to "reset" Brexit negotiations, which are due to start next week.
Meanwhile, the uncertainty caused by the general election has led business confidence to sink "through the floor", according to the Institute of Directors.
A snap poll of 700 members of the lobby group found a "dramatic drop" in confidence following the hung parliament.
The main priority for the new government should be striking a new trade deal with the European Union, according to the IoD.
Business groups such as the CBI and EEF believe the election result has weakened the hand of those wanting a "hard Brexit", which would involve leaving not just the EU but also the single market, customs union and escaping the jurisdiction of the European Court of Justice.
They favour a deal that would give British business much the same access to the rest of the EU as they enjoy now and seem to be freshly emboldened to press their case.