Commonwealth nations must not only denounce modern slavery but take the lead on global efforts to eradicate the trade by 2030 by strengthening laws, activists said on Tuesday.
They said the nations should also work with businesses and protect women and girls from being enslaved.
Leaders of the 53-country Commonwealth, mostly former British colonies representing 2.4 billion people, are meeting in London this week at a biennial summit to address shared global challenges and discuss business, youth and women's rights.
The countries should use the platform to commit to collaborate to end a crime estimated to affect 40 million people globally and that raises annual profits of $150 billion, said Urmila Bhoola, the leading United Nations official on slavery.
"Given the vast disparities between Commonwealth states, there is a need for collaboration," Bhoola, the UN special rapporteur on contemporary forms of slavery, said at a panel.
"This is an urgent imperative – lives are at stake."
More than half of the world's slavery victims are estimated to live across the Commonwealth, yet only three of its states have ratified a 2014 UN treaty to end forced labour, according to a report by the Commonwealth Human Rights Initiative (CHRI).
While 80 per cent of Commonwealth nations have criminalised human trafficking, at least half have penalties that are either too lenient - in terms of fines and jail sentences - or inhumane such as capital punishment, said the Walk Free Foundation.
"There is a huge gap in reality between the number of slavery crimes and the number of cases registered and brought to court," said India's 2014 Nobel Peace laureate Kailash Satyarthi, a top child's rights and anti-trafficking activist.
"Developing nations in the Commonwealth need more financial and technical support to improve the enforcement of their laws."
Following Britain's landmark 2015 Modern Slavery Act, India in February approved a tough new law which can jail traffickers for life, while Australia is mulling anti-slavery legislation that is expected to be tabled in the coming months.
Commonwealth countries must also focus on empowering women and girls - estimated to account for seven in 10 victims of slavery worldwide, and the vast majority of the 15.4 million people trapped in forced marriage, according to research by Walk Free.
"This makes a focus on gender crucial," said Andrew Forrest, founder of Australia-based Walk Free, adding that only 13Commonwealth countries have criminalised forced marriage.
These nations must also do more to engage with business to fight slavery, with only three having done so to date, he added.
"This is unacceptable given we know that 16 million people trapped in modern slavery globally in 2016 were exploited in the private economy," Forrest told the Thomson Reuters Foundation.