Britain’s mergers and acquisitions watchdog has waded into the merger tussle between Xstrata and Anglo American, asking Xstrata to confirm its estimates of $1bn in synergies from the proposed £40bn ($66bn) tie-up.

The request from the Takeover Panel followed representations made to it by Anglo American, which was concerned at the discrepancy between media and market estimates on the synergies from a possible merger, and the officially-stated estimate in a public letter from Xstrata last week, which estimated synergies of “over $1bn” from the deal.