"Nearly 2,500 barristers who are essential to the criminal justice system in England and Wales are starting industrial action over concerns about legal aid funding.
They will refuse to step in at the last minute to pick up court appearances or preparatory work for colleagues whose cases are over-running.
It stems from an unresolved row with the government over funding.
The government said the action would make existing backlogs worse.
The Criminal Bar Association (CBA) has accused ministers of dragging their feet over implementing a 15% rise in rates for legal aid - as recommended by an independent review.
The CBA said 15% was the bare minimum needed to stop the system from collapsing.
Figures published last week revealed 10% of criminal barristers involved in legally-aided work have withdrawn from those cases in the past year.
They also showed the government’s measures to reduce a nationwide backlog are making slow progress.
The action, supported by about 90% of criminal barristers, will begin this morning.
The informal system at the heart of this action is the legal equivalent of coming off the subs bench in the 87th minute after another player has hobbled off.
If a barrister is stuck in a delayed case in one court, meaning they can’t make their commitment in another, a colleague volunteers to read the case file - often overnight - and goes along as their substitute.
Once a judge knows both the prosecution and defence are properly represented in court, he or she can keep the case moving towards verdict.
If there’s no barrister available, judges can only cancel the hearing until another day.
Within weeks, those adjournments will be competing for slots against both new cases, as well as the existing backlog.
Worst case scenario? Entire prosecutions will be put back for months as courts struggle to find a way to get them heard.
The government is pretty furious - but many criminal lawyers - particularly the young - say they are barely making a living and there’s now too few of them left. The evidence for that? Some 280 trials were put off between October and December last year because of a shortage of prosecutors and defenders.
An independent review last year warned that courts could grind to a halt without an immediate injection of £135m into the legal aid system.
The review, launched after predictions the system could collapse, determined that ministers should find the funding to reverse a large loss of lawyers who are vital to the process.
The Ministry of Justice said it was increasing investment in criminal legal aid by £135m a year, and this included an increase in fees for criminal barristers resulting in a pay rise.
Justice minister James Cartlidge: "The Crown Court backlog is now falling thanks to our decisive action and the hard work of legal professionals and as a result of our reforms the typical criminal barrister will earn nearly £7,000 extra per year.
“This is a significant pay rise and I encourage the Criminal Bar Association to work with us, rather than pursue unnecessary disruption in the courts which will only serve to delay justice for victims.”
What is criminal legal aid?
The legal aid system in England and Wales ensures that suspects who cannot afford lawyers are properly advised and represented, from their police interview through to trial.
The system is critical to keeping justice moving by saving court time and making sure defendants get a fair trial.
However, the amounts paid for this work have been both frozen and then cut over the past 25 years.
In an independent review, former judge Sir Christopher Bellamy QC supported many of the concerns of lawyers - and urged ministers to find a minimum of £135m to stem an exodus of lawyers from criminal justice and to help tackle the current backlog of 59,000 cases in the Crown Court system.
“£135m is, in my view, the minimum necessary as the first step in nursing the system of criminal legal aid back to health after years of neglect,” he said.
“I do not see that sum as ‘an opening bid’ but rather what is needed, as soon as practicable, to enable… the whole criminal justice system to function effectively, to respond to forecast increased demand, and to reduce the backlog.”
“There is in my view no scope for further delay.”
Correction: This article has been amended to make clear that the figures show 10% of criminal barristers involved in legally-aided work have withdrawn from those cases in the past year. An earlier version wrongly stated that 10% of criminal barristers had quit in the past year." Thousands of barristers take action over legal aid - BBC News