In an exclusive interview with Sky News, Kevin Hyland expressed frustration that slavery victims were being “held on a piece of string”, waiting for overdue decisions on their status.
The government says it is leading the world in tackling slavery, but Mr Hyland is among critics who say the victims are not being properly looked after once they have escaped or been rescued.
Home Office guidelines say potential victims will be assessed and processed within 45 days of coming to the attention of authorities.
But Sky News has learned that the majority of cases go well over that time period, with many people waiting more than a year.
Speaking on national anti-slavery day Mr Hyland told Sky News: “One of the things I highlighted to the government was the fact that it was taking so long to make a decision.
“In many ways it is not the funding that is the problem it is the decision making.
“If those decisions were made in a much shorter time then that funding could be used to give the victims other opportunities such as education, work, whatever else that may be.”
The former commissioner says that none of his recommendations have been properly seen through.
He added: “We know there are problems. They’ve been identified, the solutions have been put forward and a year later we are still waiting for those to be implemented.
“I think we have to accept that’s just not good enough. We need to see things implemented quicker.”
Earlier this year the prime minister described modern slavery as “the great human rights issue of our time”.
On a trip to Africa she announced measures to tackle the problem of trafficking.
But charities also say victims are not being properly supported and some even fall back into slavery after being rescued due to delays in getting help.
The charity HESTIA, which helps find safe houses for hundreds of victims every year, says the average wait for a decision is 303 days.
During this time victims are not allowed to work and are unable to get on with their lives.
Ella Read, area manager for HESTIA, said: “There is a constant background anxiety of them knowing that any day a decision could come through the post.
“It could be a decision in their favour, or it could not be a decision in their favour, and that can really hamper someone’s ability to meaningfully recover in that time.”
In 2014 a Home Office study estimated there were between 10,000 and 13,000 modern slaves in the UK.
But the Walk Free Foundation recently suggested there could be as many as 136,000.
More than 5,000 victims of slavery and trafficking were referred to the police in the last year alone. That is a 35% increase on the year before.
Sky News interviewed one 48 year-old victim who escaped from sex slavery and has waited 18 months for a decision on her status.
“Susie” was brought to the UK from Ghana by a man she thought she was going to marry. Instead, the man confiscated her passport, she was kept in servitude and forced to have sex.
Sexual exploitation is the most common form of modern slavery reported in the UK, followed by labour exploitation, forced criminal exploitation and domestic servitude.
A Home Office spokesman said: “Modern slavery is a barbaric crime that destroys the lives of its victims, which is why we introduced the world-leading Modern Slavery Act in 2015 and have put in place the Modern Slavery Strategy.
“The package of reforms we’ve announced to the National Referral Mechanism will markedly improve our system for identifying and supporting victims and lead to quicker, more certain decisions on cases.
“We aim to make a decision as quickly as possible, but we need to ensure we have all the information available, which can take time in complex cases.
“Potential victims receive a comprehensive package of support while a decision on their case is being made.”