The UK holds ties with four of the world's worst corporate tax havens, according to a study by Oxfam.
The charity has pinpointed Bermuda, the Cayman Islands, Jersey and the British Virgin Islands as serious offenders when it comes to offering unproductive tax incentives and extremely low corporate tax rates.
It warned that the UK risked losing its reputation "as a global force for good" if it failed to tackle tax avoidance on overseas territories and crown dependencies.
Ana Arendar, Oxfam's head of inequality, said maintaining the status quo would rob countries of the money they needed to pay for education and life-changing treatment.
"As long as tax havens are able to help rich clients and big businesses avoid paying their fair share, those who can least afford it are left picking up the tab," she said.
"Allowing our overseas territories and crown dependencies to operate as tax havens undermines Britain's efforts to be an outward-facing, responsible member of the international community.
"It's time to end this embarrassing contradiction in our own backyard."
Oxfam said developing countries lost out on 138 billion US dollars (£110 billion) a year because they were forced to offer competitive tax incentives to attract multinational companies.
It also hit out at moves to slash corporation tax to help shore up Government finances, claiming that it often triggered a VAT hike which could punish the poorest people in society.
Prime Minister Theresa May revealed her ambition last month to ensure the UK had the lowest corporate tax rates in the G20 group of advanced economies.