Rio 2016: Michael Phelps wanted more anti-doping tests

Chen Xinyi is the first swimmer in the Rio Olympics to fail a doping test

Michael Phelps was among a group of 23 American swimmers who demanded more drugs testing in the run up to the Rio Olympics, the BBC has learned.

The Olympics' most decorated athlete signed a letter sent to the International Swimming Federation (Fina) in December.

It proposed carrying out "at least six" targeted, out-of-competition tests on the 10 quickest swimmers in each of the 34 Olympic events.

Fina accepted the proposal in January.

The tests for a range of prohibited substances include the collection of urine and blood samples and represent a significant increase.

Other US swimmers making the "urgent request" to the governing body included Ryan Lochte and Missy Franklin.

The swimming competition in Rio has been marred by increased tension between competitors after the IOC resisted demands for a blanket-ban of Russia despite the McLaren report into state-sponsored doping.

Seven Russian swimmers have been allowed to compete in Rio after initially being banned, leading to complaints from several rivals.

American Lilly King and Britain's Chloe Tutton have both appeared to question the involvement of Russia's Yuliya Efimova, who was banned in 2013 after traces of an anabolic steroid were found in her system.

Australian 400m freestyle champion Mack Horton called Sun Yang "a drug cheat" after his Chinese rival served a three-month ban in 2014 for testing positive for a banned substance.

China's Chen Xinyi became the first swimmer to fail a doping test in Rio. The 18-year-old tested positive for diuretic hydrochlorothiazide on Sunday - the day she finished fourth in the women's 100m butterfly final.

The letter was signed by 23 American swimmers

The letter sent by the American swimmers was also signed by Camille Adams, Nathan Adrian, Michael Chadwick, Tyler Clary, Kevin Cordes, Conor Dwyer, Matt Grevers, Jessica Hardy, Chase Kalisz, Caitlin Leverenz, Simone Manuel, Katie Meilli, Cody Miller, Lea Neal, David Nolan, Alison Schmitt, Josh Schneider, Tom Shields, Austin Surhoff and Kelsi Worrell.

It read: "We have learned from the Wada [World Anti-Doping Agency] independent commission that state-sponsored doping in Russia involved athletes from sports other than just athletics, including swimmers.

"We also know there are a number of top swimmers from other countries where there has been a history of doping, and many would question the effectiveness of their national anti-doping organisations.

"According to the clean swimmers of the world, there is no more important priority for the use of Fina funds."

Fina spent £560,000 on anti-doping in 2015 - approximately one tenth of the figure that cycling's governing body, the UCI, spent.

They have also had swimmers test positive in recent years, particularly Russia, which hosted the Fina World Championships in Kazan last year. Five young Russian swimmers failed tests in late 2013, with three more cases since then.

Fina President Julio Maglione has claimed that Wada "exceeded their power" when compiling the McLaren report into Russian doping.