Qatari developer Barwa paid Land Securities £250m for the landmark Oxford Street building site last month.

Now the property firm, chaired by a Qatari businessman at the heart of a legal row with Prince Charles, is putting pressure on Mace to reduce its building costs.

Mace started work on Park House in May under a deal thought to be worth around £120m.

The 500,000 sq ft Park House project, the largest development on Oxford Street for 40 years, is being delivered under director Jonathan Foster by the same Mace team that built the Ropemaker Place project .

Original developer Land Securities estimated the scheme’s final value to be around £400m when it signed a contract with Mace.

A source close to the project told the Enquirer: “The Qataris are demanding that the build costs be brought down.

“Clearly there will be some tough talks ahead on this. It’s an interesting issue because Mace holds a contract that was signed with the previous owners Land Securities.”

A spokesperson for Mace said: “We do not and will not comment on speculation about contracts we are working on.”