A row over the ownership ratio in the mooted merger between Airbus parent EADS and Britain’s BAE Systems is the latest threat to a merger which would create the world’s biggest defence and aerospace group.

Sources told Reuters on Tuesday a 60-40 split in favour of EADS was being questioned on several sides and talks were under way about possibly adjusting it. Both companies issued denials.

Europe’s biggest defence deal in a decade faces several hurdles including how to ringfence top-secret projects and how to satisfy governments keen to safeguard EADS operations in France and Germany.

With an October 10 deadline to decide whether the talks proceed, EADS Chief Executive Tom Enders, one of the merger’s main architects, faces the German parliament on Wednesday to put forward his case.

Yet two sources familiar with the transaction told Reuters that the deal, which the firms acknowledged publicly on Sept 12, already faced differences on how the two would be valued.

They said EADS and BAE were discussing valuing EADS at more than the 60 percent originally announced, pushing the ratio to potentially 63:37 or even 65:35.

“The ratio could be changed up to 3 percentage points in favour of EADS,” one EADS adviser told Reuters.

EADS and BAE issued a denial: “The joint announcement published on 12 September outlining the 60-40 shareholder split is the only valid statement on this subject”.

It said “any speculation suggesting a different ratio is inaccurate”.