A new statutory duty to report potential victims of trafficking, included in the modern slavery bill published on Monday, is expected to bring help to an extra 1,500 victims in Britain every year, the Home Office has said.

Police, immigration and other public sector staff will be required to report all potential victims of trafficking with whom they come into contact to the National Crime Agency. The victims themselves will be able to remain anonymous and will not be obliged to accept support they do not want.

The duty is a key part of the draft modern slavery bill published by the home secretary, Theresa May, designed to tackle the hidden nature of the crime, which she described as going "unseen for months and years … in quiet cul-de-sacs and streets and roads around the country".

May said victims were held against their will and forced into a life of abuse, servitude and inhumane treatment. "We can no longer allow men, women and children of all races, cultures, nationalities and ages to go unseen. We need to take action."

The main elements of the draft bill include:

• Consolidating and simplifying existing slavery and trafficking offences.

• Increasing the maximum sentence available from 14 years to life imprisonment.

• Creating a new anti-slavery commissioner.

• Establishing a legal duty to report potential victims of trafficking.

• Introducing slavery trafficking prevention orders and slavery and trafficking risk orders to restrict the activities of those who pose a risk and those convicted of trafficking offences so they cannot cause further harm.

A Home Office impact assessment published alongside the bill says evidence from the UK Human Trafficking Centre suggests there are 2,255 possible victims of human trafficking in Britain in the UK. This is below the 10,000 estimated figure cited by the Labour MP Frank Field, who was asked by the home secretary to publish his own study of the problem.

The Home Office white paper that includes the draft bill says 1,186 potential victims from more than 90 countries were referred to the authorities in 2012. The largest single group – 205 victims – came from Nigeria, followed by 135 from Vietnam and 128 from Albania.

The home secretary says legislation is not the only answer to the problem. She intends to set up a modern slavery unit in the Home Office, work with other countries to tackle the problem at source and publish an action plan in the spring that will set out a comprehensive response across government to increase awareness in communities and in business.

May has promised that the draft legislation will reach the statute book before the general election in 2015. She has asked Field to lead a series of evidence sessions as part of the pre-legislative scrutiny of the measures.

Most of the measures in the draft bill are regarded as uncontroversial at Westminster, with MPs who have voiced concern at the issue urging the home secretary to go further.

Field's review published on Monday suggests the legislation should include a measure to place a legal duty on companies to outline the steps they have taken to eradicate slavery, so that consumers can be reassured that workers are not being exploited.

However, the Home Office white paper says the government intends to continue to work with businesses on a voluntary basis so they can ensure that their workforces and supply chains are not exploited.

The government has also declined to endorse moves advocated by Field and the shadow home secretary, Yvette Cooper, towards the introduction of a pilot guardianship scheme under which a trained adult would be given the task of looking after child victims.

Field said: "The report encourages the government to make the most of the opportunity that it has created for itself. The home secretary, by establishing this review, has shown that her bill will be informed by our recommendations, and signals how serious her intent is. I urge her to 'reach for the stars' in creating a bill which delivers all that 'world leader status' promises."