The UK's first anti-slavery chief has resigned after four years in the job.
Commissioner Kevin Hyland says he has been frustrated by government interference in his role.
He described the world-leading law as a "game-changer in many ways".
However, he acknowledged criticism from anti-trafficking campaigners about a lack of convictions, mixed support for victims and limited action from businesses to address slavery.
When was he appointed?
In 2014, as part of the UK's widely-lauded Modern Slavery Act.
The former police officer has been praised for helping to enact a landmark law and spearhead a global drive to end modern slavery by 2030.
So what is the problem?
Hyland says he has been frustrated by the government interfering with a role intended to be separate from the state.
He has declined to elaborate but has called for autonomy for his successor. The call has been echoed by leading anti-slavery activists.
"As the inaugural incumbent in a unique role, there have predictably been some learning points for all around the precise nature of the independence set by the founding legislation, but I leave the role confident that my successor can only benefit from this learning," Hyland said in a statement.
Promises and frustrations
Hyland's term was only extended by a year after the Home Office said the commissioner was the subject of an inquiry following a query about his conduct by a charity running a modern slavery helpline.
Has there been any response from the government?
Yes. UK Prime Minister Theresa May has praised Hyland for his service, saying he should be proud of his achievements.
"As the first incumbent of the role, you made a significant contribution to shining a spotlight on the scale and nature of modern slavery in the UK and internationally," she told him.
How much of a problem is modern slavery?
Globally, it is estimated more than 40 million people are thought to be trapped in forced labour, forced marriages and sexual exploitation.
In the UK, at least 13,000 people are estimated to be victims of modern-day slavery. Police think the true figure is much higher.
How did the Modern Slavery Act help?
It introduced life sentences for traffickers, measures to protect people at risk of being enslaved and made large companies scrutinise their supply chains for forced labour.
What they are saying
"As independent Anti-Slavery Commissioner, Kevin has always argued for greater resources that have often been promised. He has also argued for a more co-ordinated approach to combating this terrible crime," Cardinal Nichols, the head of a global alliance of the Catholic Church and law enforcement officials set up at the request of the Pope.