The ex-Watford boss wants to focus on a return to club football.

Boothroyd said: "It's been the greatest privilege to represent the FA and lead England teams into high-profile tournaments.

"While we didn't finish in the way we wanted, I'm very proud to have been able to work so closely with players who I know will go on to have remarkable international careers and to have experienced some very special moments with them over the years.

"I'd like to thank my staff and all of the incredible support team that have done so much to help us remain competitive against other elite nations.

"In every single get-together they embraced our unique challenge of rightly prioritising the needs of the senior team while ensuring we gave our all to try and get a positive performance and result."

Boothroyd – who won the Toulon Tournament with England in 2018 – stepped up from Under-20s manager to the Under-21s to replace Southgate in 2016, when Southgate was appointed interim senior boss after Sam Allardyce's exit.

He took the job permanently in 2017 and guided the Young Lions to the semi-finals of the European Championship in Poland, losing to Germany on penalties in the semi-finals.

Two years later, in Italy and San Marino, England failed to get out of the group – losing to France and Romania and drawing with Croatia.

Last month the Young Lions were knocked out in the group stages of Euro 2021 in Hungary and Slovenia.

Defeats to Switzerland and Portugal meant they needed to beat Croatia by two goals to reach the knock-out phase. They were leading 2-0 going into injury time only for Domagoj Bradaric's last-gasp goal to eliminate them. 

Speaking before the Croatia game, Boothroyd described managing the Under-21s as "the utterly impossible job", citing the competing demands of winning games and developing players to step up to the senior team.

Southgate said: "I'd like to thank Aidy for his wholehearted support and dedication to developing English players and coaches across the last seven years.

"His pride in working with all of our development teams has shone through and his support of the senior team has been greatly appreciated. I wish him every success on the next step of his career."

The FA will now start the search for Boothroyd's successor – with Frank Lampard, Justin Cochrane and Paul Simpson linked.