Mrs May said the money would be targeted at dealing with the routes of the people trafficking trade in countries like Nigeria, as she unveiled a new taskforce which is to co-ordinate the Government response to slavery.
Ministers estimate there are between 10,000 to 13,000 potential victims of slavery in the UK.
A review by barrister Caroline Haughey to mark the first anniversary of the Modern Slavery Act found that 289 modern slavery offences were prosecuted in 2015, and there was a 40% rise in the number of victims referred for support.
The Government has also asked Her Majesty's Inspectorate of Constabulary (HMIC) to assess the police response to modern slavery.
Mrs May said: "This Government is determined to build a Great Britain that works for everyone and will not tolerate modern slavery, an evil trade that shatters victims' lives and traps them in a cycle of abuse.
"Last year I introduced the world-leading Modern Slavery Act to send the strongest possible signal that victims were not alone and that those responsible for this vile exploitation would face justice.
"We must do more and the historic £33.5 million funding will allow us to go even further to support victims.
"Alongside this, the Haughey and HMIC reviews send a clear message that the criminal justice system must ensure that perpetrators have nowhere left to hide. I am pleased to see progress but we will not stop until slavery is consigned to the history books."
Ms Haughey, who has prosecuted a variety of servitude, slavery and human trafficking cases including the first case of modern slavery, said: "The Modern Slavery Act has set an international benchmark to which other jurisdictions aspire. The Act itself is fit for purpose and our priority should be to maximise the impact of the provisions that came into force a year ago.
"One year on, law enforcement agencies are using the powers in the Act and the number of prosecutions and of victims supported has increased. But this is a generational fight to protect the vulnerable and voiceless and I believe we need to do more.
"There is a lack of consistency in how law enforcement and criminal justice agencies deal with the victims and perpetrators of modern slavery. We need better training, better intelligence and a more structured approach to identifying, investigating, prosecuting and preventing slavery, including learning from what works and what does not."
The move came as the UK's anti-slavery commissioner warned that people trafficking cases are not being investigated properly.
Kevin Hyland expressed concern at the levels of potential slavery incidents being recorded as crimes.
"What's alarming about that is that we do have people reporting to the authorities, but then they are not being properly investigated.
"The real concern that I have is that in 2015 we had 986 cases involving minors, yet the official figures show that there's only 928 actual crime recorded incidents. So that means ... potentially the cases involving minors are not being investigated properly.
"The fact that the number of crimes recorded hasn't even reached the number of minors actually tells me that all these cases ... they are not being investigated properly because until they are recorded properly, and there are the resources put into it to see what the circumstances are behind that, we can't be satisfied that we can say it is, or it isn't, a crime of modern slavery," the commissioner told the BBC.
The global trade in people is estimated to cost £113 billion, with human trafficking for sexual exploitation believed to cost the UK £890 million.
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