LONDON (REUTERS) - Missy Franklin, the 2012 World Swimmer of the Year, announced her retirement on Wednesday (Dec 19) at age 23, citing intense pain from shoulder injuries that have dogged her for nearly three years.

Franklin was a 17-year-old when she won the 100 and 200 metres backstroke events at the 2012 London Olympics, the latter in a world-record time that still stands more than six years later.

She also made the final in two other individual events in London, and was part of two victorious US relay teams.

Franklin picked up six golds at the 2013 Barcelona World Championships, but never improved her personal best times from London.

She struggled to replicate her teenage performances after first experiencing bad shoulder pain in early 2016.

She still made the US team for the Rio Olympics, however, where she picked up another relay gold after swimming in the heats, before having surgery on both shoulders last year.

"This was perhaps the hardest letter I've ever had to write... Today, I announce my retirement from competitive swimming," Franklin wrote on Twitter.

She explained her reasons for retiring in an article for ESPN.com.

"I've been in too much pain, for too long, to go through another surgery with a longer recovery time and no guarantee it would even help," she wrote.

"It took me a long time to say the words, 'I am retiring.' A long, long time. But now I'm ready."