The Prime Minister hosted senior industry figures from across the continent in Downing Street to hear their concerns.
Emma Marcegaglia, president of the lobbying group BusinessEurope, said it was crucial for progress to be made within two weeks in order for trade talks to be given the green light by EU leaders at December's summit.
She told reporters in Downing Street: "The message is that as businesses all over Europe we are strongly united.
"We are very concerned for the fact that we don't really see progress in the negotiation and we know that there are two weeks to get to sufficient progress so that in the council in December it can be decided to go on to phase two.
"So, these two weeks are absolutely important.
"We need to have a transitional period where the UK stays in the customs union and single market because companies need certainty."
She added: "We don't want uncertainty, we are very concerned. We know that if companies don't see certainty probably they will have a contingency plan and probably they will leave the UK, or they will invest less.
"So, these two weeks are extremely important."
More to follow...