When Donald Trump unveiled a helipad at the White House on Monday, his remarks were barely audible in live footage of the event.

This was due to a row over an institution that rarely makes the news – the White House Press pool.

Yet it is at the heart of relations between the US president and the media, and this dispute could have constitutional implications.

Coverage of upcoming presidential engagements, which include the UN General Assembly in New York, could be affected.

What is the White House press pool?

It is a group of news organisations that combine resources to follow presidents wherever they are - in the Oval Office, at state dinners, on Air Force One or on holiday.

There are separate pools for TV, photographers and print and radio outlets, as well as a "new media" category.

The idea, conceived decades a...