More than 150 business leaders have called on South Yorkshire's four councils to "push ahead" with the £900m devolution deal first tabled in 2015.
They say further delay to signing the agreement was of "real concern" and "potentially very damaging".
The deal was put in doubt when 17 councils across Yorkshire, including Doncaster and Barnsley, said they were exploring a possible county-wide deal.
The BBC has contacted the four councils for comment.
Under the proposed deal an elected mayor would be responsible for transport budgets and strategic planning and have access to £900m of government money over the next 30 years.
In a letter to Sir Steve Houghton, Ros Jones, Chris Read and Julie Dore - the leaders of Barnsley, Doncaster, Rotherham and Sheffield councils respectively - the group warned many business leaders were "very concerned about reported further delays".
'South Yorkshire first'
It said: "The possible loss to the region of significant funding is clearly of real concern; the damage to the region's reputation and impact on business confidence is potentially equally or more damaging to our long term prospects.
"A Yorkshire deal may emerge as a possibility in the years ahead, but surely we will be better placed to be a strong component of that, should it arise, if we push ahead to really strengthen South Yorkshire first."
The group also believe the appointment of an elected mayor would "enhance our clout as a region".
Analysis: James Vincent, BBC Look North political editor.
Privately the government has been warning South Yorkshire it needs to get on with sorting their devolution deal out.
It has said that if the southern part of the White Rose does not go ahead with devolution it will bear the costs of the deal so far and might even end up with a mayor without any powers.
Now, Yorkshire has come up with the bright idea to unite the whole county; except not all of the county is on board.
As always money is pressing the issue.
Businesses in South Yorkshire fear that without the South Yorkshire deal investment and opportunity will be lost.
They want it signed and sorted before devolution is widened to all of Yorkshire,
Doncaster and Barnsley are key here.
They're signed up to the pan-Yorkshire idea and are not making positive sounds about South Yorkshire devolution.
Devolution was originally announced as a deal for the Sheffield City Region - which consists of the four South Yorkshire councils as well Bassetlaw, Bolsover, Chesterfield, Derbyshire Dales and North East Derbyshire - however the five non-Yorkshire councils have since backed away.
Doncaster Council declined to comment on the letter while Rotherham Council leader Chris Reed said he would "consider our options and work towards the best deal for South Yorkshire".
Speaking when the pan-Yorkshire deal was mooted Sir Steve said: "The argument goes, the bigger the deal the more money and more power you are likely to get."