The multiple world and Olympic champion made the announcement on The Chris Evans Breakfast Show on Virgin Radio this morning.

Sir Mo, who supposedly retired from the track in 2017 to concentrate on road running, told Evans: "To be clear, I'm concentrating on the 10,000m for Tokyo 2020, and then after that have fun with a marathon and do what I need to do."

Now 36, Sir Mo - who won the Olympic 10,000m titles at London 2012 and Rio 2016 - finished third in the 2018 London Marathon and won the Chicago Marathon, but this year finished fifth in London and only eighth in the Chicago race last month, more than four minutes adrift of the winner, Kenya's Lawrence Cherono.

Explaining his decision, Sir Mo added: "After the Chicago Marathon, I've taken a nice break with my family, just enjoyed being around my kids.

"And now, for a little while, I was just thinking about Tokyo 2020 - it's not too far away.

"I've really enjoyed the marathon, but having competed in the 10,000 before at London and then Rio, in Tokyo 2020 I would like to compete in the 10,000m.

"So, I'll come out of retirement and compete back on the track.

"One of the reasons is you watch telly and watch the people - I was watching the World Championships in Doha.

"I watch the 10,000m and other races and part of you gets excited, just like seeing people winning medals, for your country and stuff, and you just ask yourself.

"It almost felt like I needed to be there.

"I still believe and I've still got that drive and hunger and all I want to do is win races and do what I do best, but at the same time enjoy it.

"And I have enjoyed the marathon and I'm not finished with the marathon.

"But in Tokyo I'd like to go out there and see what I can do for my country in the Olympics. 

"After I retired in 2017, I said, I'll step aside and I'm done - then the following year I'm like, 'Hmm' a little bit.

"And now it has been two years, I want to get back out there.

"A marathon has been tough because you can only race every six months once, then you have to wait a whole six months.

"It's just different."

The news of his decision comes at the end of a difficult year for Sir Mo, who has been caught in the backwash of the United States Anti-Doping Agency's four-year suspension on doping charges of Alberto Salazar, the coach who guided him to his Olympic titles and whom he left in 2017.

Sir Mo, who has also won six world titles over 5,000m and 10,000m, is now odds-on with the bookies in London to win a third consecutive Olympic 10,000m title.

Sir Mo is also quoted at 10-1 to win a "triple double" of 5,000 and 10,000m in Tokyo.

He is 2-1 to win medals in both events.