The Foxes - currently second in the Premier League - come to St. James' Park on New Year's Day (kick-off 3pm GMT).
United's final game of 2019, at home to Everton, was also a sell-out, with a crowd of 52,211. It was the first game of the club's new ten-match half-season ticket.
Following discussions between head coach Steve Bruce and managing director Lee Charnley, the club decided to offer current season ticket holders a free additional half-season ticket to help get a full, vibrant stadium behind the team.
To purchase tickets for future matches, please visit book.nufc.co.uk, call 0344 372 1892 or visit the St. James' Park box office (Milburn Stand) in person.