The London Stock Exchange has warned its proposed £20 billion tie-up with Germany's Deutsche Borse could be scuppered if Britain leaves the European Union.
The bourses stated that a vote for Brexit "would put that project at risk", but said the result of the referendum was not a condition of the potential merger.
The concerns came as they put further flesh on the bones of the proposed deal, revealing LSE chief executive Xavier Rolet would step down if it gets the go-ahead.
The two stock markets said they have now formed a referendum committee to assess the outcome of EU referendum on June 23.
They said: "The parties are proceeding on the basis that existing regulatory and political structures remain in place.
"It is recognised that a decision by the United Kingdom electorate to leave the European Union would put that project at risk," they added.
However, the bourses said the potential deal could "serve global customers irrespective of the outcome of the vote", but it may impact "the volume or nature of the business conducted in the different financial centres".
The fresh attempt to create one of the biggest exchange companies in the world was launched on Tuesday.
It comes after previous moves to join forces failed in 2000 and 2004-5 when talks collapsed.
The new deal on the table would be an all-share merger, with LSE shareholders holding 45.6% of the group and Deutsche Borse the remaining 54.4%.
It would see Deutsche Borse boss Carsten Kengeter becoming chief executive of the combined company and LSE's Donald Brydon taking up the role of chairman.
The current chairman of Deutsche Borse Joachim Faber would become deputy chairman and senior independent director, with LSE's David Warren retaining his position as chief financial officer.
The combined business will run headquarters in London and Frankfurt, the bourses said.
The London Stock Exchange is one of the world's oldest stock exchanges and can trace its history back more than 300 years.
The wider LSE Group was formed in October 2007 when the London Stock Exchange merged with Milan stock exchange Borsa Italiana.
It has completed a series of deals since then, the biggest of which was its £1.6 billion takeover of American stock index and asset management business Frank Russell in 2014.