More than 1,000 UK business leaders now back a renegotiation with the European Union and a referendum, according to Business for Britain (BfB).
The campaign group announced that it has signed up more than 1,000 members – just a year and a half after its formation.
The organisation calls for a "better deal" from the EU for the UK and also supports an in/out referendum on the country's membership of the economic and political union.
The Prime Minister David Cameron has promised that a Conservative government would hold a referendum in 2017.
"The growing number of signatories to BfB shows that business leaders are firmly behind the policy of getting a better deal for the UK through EU reform and backing up that process with a referendum," said Alan Halsall, co-chairman of BfB.
"It's vital that voters are given a say on Britain's membership of the EU to ensure that the process of reform and renegotiation has real teeth.
"We need the EU to be more competitive, to trade with more emerging markets across the globe, and to help, not hinder, our long term economic future."
The announcement comes after the Chancellor George Osborne claimed he secured a
"victory" by getting the EU to push back the UK's deadline for a £1.7bn ($2.7bn, €2.1bn) tax bill and accept "just half" the original surcharge.
Osborne said the move means that the bill from Brussels will not have to be paid until 1 September 2015, instead of 1 December, and the UK will pay a total of £850m.
However, a series of other European finance ministers, who attended the meeting that Osborne was in, insisted that the UK has had no discount.