Mr McDonnell visited West London’s independent builders’ merchant Lords in his constituency of Hayes and was reminded by the company how merchants – builders, timber and plumbers merchants – serve the construction industry.

Discussions included the Mayor of London’s draft London Plan, which has a target of 66,000 new homes a year.

“Lords is pleased that housing remains at the top of the political agenda and that the shadow chancellor supports moves to build many more new homes,” said Shanker Patel, Lords’ chief executive.

“Merchants want to see unstinting efforts by national government in Whitehall, and local authorities in London, to close the gap between housing demand and supply.”

But he warned that boroughs would struggle to deliver these homes on available brownfield land.

“The worry is that railway depots and other sites used for merchants’ yards and minerals’ handling will be lost,” he added, saying London’s industrial sites should be safeguarded, otherwise freight would be pushed further out of London.

Last year Lords’ Dawley Road branch was completely rebuilt to make way for a larger trade counter, vehicle management hub, and new group training suite.

Mr McDonnell’s visit was arranged by the Builders Merchants Federation (BMF) as part of its industry and parliamentary programme.