The Evangelical Alliance's general director has signed a letter calling for the Prime Minister to take action to ensure slavery-free supply chains in the UK.

To coincide with the Modern Slavery Bill's passage through Parliament, the letter welcomes the introduction of a legal requirement for companies to report annually on their steps to address slavery in supply chains.

But joining with representatives from Unseen, CAFOD, CORE, Traidcraft and others, Steve Clifford has urged the government not to "evade their responsibility to use the Modern Slavery Bill to set a clear world-leading standard in the UK and beyond for how companies should deal with this very pressing issue".

"The Modern Slavery Bill has the potential to be one of the most far-reaching laws of your government, however, it needs further amendment to be workable and truly effective and deliver support to UK businesses," the letter says.

The Transparency in Supply Chains clause of the Bill is already attracting international attention, with other legislatures looking closely at how the UK plans to tackle slavery in supply chains.

Historically, Britain has played a significant role in combatting slavery and the letter's signatories say it is time to do this again by delivering the world-leading legislation that the Government has promised.

The Bill left Committee in the House of Lords with many plaudits but also with cross-party consensus that the clause on supply chains needs to be clearer and more specific.

The letter delivered to David Cameron reads: "We are already working with Government departments, businesses, the investment community and civil society groups to challenge abusive supply chain practices.

"On the basis of this experience, we recognise that more detail is needed on the face of the Bill, including a set of minimum criteria that specifies what a company must actually do to comply with the proposed measure. It also needs to be clear which companies will have to report."

In order for government, investors and civil society to know which companies are taking effective action, the reports need to be available at a single site and comparable between sectors, claim the alliance of organisations.

"This transparency measure needs to be coupled with adequate enforcement mechanisms to deliver change, including a link to the Directors' Report to ensure that company directors take the provision seriously."

To conclude, the organisations write: "The opportunity to legislate on such far-reaching issues does not occur frequently. The nature and scale of slavery and forced labour create an urgent imperative on Government to get this part of the Bill right first time.

The letter was sent on behalf of the Alliance's general director Steve Clifford, along with Dr Aidan McQuade, director of Anti-Slavery International, Chris Bain, director of CAFOD, Marilyn Croser, director of CORE Coalition, Kumar Swamy, director of Dalit Freedom Network UK, Steve Trent, executive director of Environmental Justice Foundation, Caroline Robinson, policy director of Focus on Labour Exploitation (FLEX), Linda Devereux MBE, chair of trustees of Homeworkers Worldwide, Terry Tennens, chief executive of International Justice Mission UK and Samantha Maher, policy director of Labour Behind the Label.

Paul Parker, recording clerk of Quakers in Britain, Catherine Howarth, CEO of ShareAction, Mags Vaughan, chief executive of Traidcraft, Andrew Wallis, CEO of Unseen, Ruth Tanner, director of campaigns and policy of War on Want and Kate Allen, director of Amnesty International UK also signed the letter.

Pictured: Steve Clifford and Dr Rachel Jordan wearing an ethically-made T-shirt by slavery-free clothing company Visible.