The 22-year-old is poised to become the second United player to join the Lions on loan following on from their capture of England under-21 central defender Charlie Cresswell, who joined on a season-long loan earlier this month.

Shackleton, who was left out of Jesse Marsch’s squad for Leeds' pre-season tour of Australia, had been linked with moves to other Championship clubs, including Reading.

But Millwall have won the race to sign the former England under-20 player, who made his first-team debut in a 4-1 win over Derby in August 2018.

Jamie Shackleton. Picture: Getty Images.

The London club are understood to be covering a large proportion of the wages of Shackleton.

Should the move go well, the deal could be turned into a permanent arrangement.

Several other squad players have left Leeds for pastures new so far this summer.

Striker Tyler Roberts has linked up with QPR and left-sided player Liam McCarron has joined Stoke City on a permanent deal.

Leeds United transfer latest: Jamie Shackleton set for medical at Championship club Millwall