The former Great Britain star opted not to contest the charge brought against him by the UK Anti-Doping agency earlier this week after providing a blood sample that tested positive at pre-season training with his new club last November.
After admitting his guilt – on the same day golf star Tiger Woods offered his own apology for his particular misdemeanors – Newton, already provisionally suspended by the RFL, will not now require a B sample test result or a hearing.
He will instead await the sentence delivered from the UKDA, almost certainly a two-year ban.
No details of the substance had previously been made public but – via a statement – Newton revealed he had taken a self-administered human growth hormone (HGH). The former Leeds, Bradford and Wigan star said: "I have made a grave error of judgment in taking a banned substance.
"I hope that, if nothing else, my stupidity will be a warning to any other professional in any sport of the consequences of doing so.
"I received notification of a positive test for a self-administered growth hormone on February 17.
"I have formally accepted via my solicitors the findings of the UKAD and consequent punishment that I will receive."
Newton continued: "I wish to formally apologise to my family, friends, colleagues and staff at Wakefield Trinity Wildcats, the club's supporters and the game of Rugby League.
"I would like to thank the Wakefield Trinity Wildcats for their understanding of the position at this time. Whilst they are extremely upset at my actions, they have been very professional in their handling of the situation.
"I support the work done by the Rugby Football League in association with the World Anti Doping Association."
Approaching his 32nd birthday, Newton's golden career is likely to be finished.
One of only five players to have featured in all 15 seasons of Super League, he was just three months into a two-year contract with Wakefield after leaving Bradford but is almost certainly facing the sack.
He had played in two of their opening three games as Wildcats emerged as surprise early-season leaders but the player, capped 15 times by Great Britain and a Challenge Cup winner with Leeds and Wigan, will now be contemplating a life outside of the sport.
HGH – the drug mentioned by Widnes coach Paul Cullen when he suggested substance abuse was rife in the game on Wednesday night – is a powerful anabolic hormone that occurs naturally in the body but a synthetic form can be injected in an effort to stimulate muscle growth and aid recovery from injury.
UKAD are expected to shed more light on the substance at a media conference on Monday