The figures, given to the BBC's 'File on 4' programme via a Freedom of Information request, show the number of officers waiting to start between February 2018 and January 2019 doubled from 169 to 350 - costing an estimated £10 million a year in wages.
In the summer of 2016, there were just 11 people 'on hold'.
"We need to be getting that time length down very dramatically," Mr Williamson told Forces News.
"That's why the Royal Air Force have been looking at new ways to bring in extra capability and extra capacity in order to ensure that people are not waiting so long.
"It is a priority, it's very important for the Royal Air Force to deal with."
In response to the latest revelations, the Defence Secretary Gavin Williamson said the RAF will be held accountable.
"We expect to see improvements over the coming year and we'll be holding the Royal Air Force to account."
Mr Williamson continued: "We're seeing improvements all the time. Putting extra capacity into the system, this is something that those officers that had been training will be seeing the benefit of.
"But it's making sure that when they're in the Royal Air Force when they've joined up they're really being made proper use of and that's something that we're continuing to continuously develop their career, so they're not on standstill," Mr Williamson added.
Sources told the BBC that pilots are yet to begin training due to a lack of planes and instructors, with some new aircraft requiring 'costly modification' before they can fly.
The sources, who wished to remain anonymous, claimed that US trainer jets bought for training at RAF Valley cannot be flown over water and that Hawk T1 jets from the 1970s are still being used to fill the gaps.
A Ministry of Defence (MOD) spokesperson said they are "utilising capacity on the Hawk T1" and "outsourcing Multi Engine training to train up to 100 Multi Engine students during a 3-year period".
The figures obtained show it now takes an RAF fast jet pilot up to seven-and-a-half years on average to start operational training, something the Commons Defence Committee described as "ridiculous and unacceptable".
The training should only take three years.
The BBC also reports that trainee pilots spend their time doing office jobs rather than flying.
The MOD admits the number of officers on hold is greater than it would expect but also said most trainee pilots will "experience a hold at the start of their career before and during flying training" and that they are given other roles to expand their skills.
The Military Flying Training System (MTFS) is run by a private consortium, Ascent.
"It's a huge contract and it's fundamentally failing," one source told the BBC. "There are so many elements that aren't working.
"It's not doing justice to the young trainee pilots. They do initial officer training and then everything stops for at least a couple of years."
The MOD said MFTS is the "biggest transformation of UK military aircrew training in a generation", and once it is fully complete, it will offer "world-class" training.
They added: "The transition to the MFTS is well underway, with student aircrew beginning training and throughout increasing as the new aircraft, simulators and other state-of-the-art training aids bed in."
The MOD said the pilot training pipeline is "actively managed" and that there is enough aircrew for frontline commitments.