A final construction contract on the 312m-high Shard is unlikely to be agreed before the end of the year, it is understood.

Developer Sellar Properties is in talks with Mace over a fixed-price deal valued in the region of £350m for the London Bridge tower. The contractor was originally expected to put in its price this summer.

However, sources close to the project say that a final deal is unlikely to be signed before Christmas owing to uncertainty in the markets, although work on the site will continue in the meantime.

Meanwhile, it is understood that Brookfield is still about a month away from submitting its price for the Pinnacle, and that a deal to build that London tower also may not be signed until early next year.