Brexit could also inadvertently harm citizens abroad, since ministers may fail to press foreign nations on their human rights records in their rush to agree trade deals, the organisation said.
Amnesty’s 409-page annual report on human rights around the world says the decision not to convert the EU Charter of Fundamental Rights into domestic law via the withdrawal bill means British citizens will be stripped of protections.
The rights of EU citizens were established at different times and in different ways and the Charter was designed to summarise all the personal, civic, political, economic and social rights in one document.
The Government decided not to convert the Charter into UK legislation after Brexit, claiming the “substantive law and the principles which underpin the Charter” can be continued without it.
Amnesty was also critical of the Government’s decision not to include in the Withdrawal Bill the ability of an individual to bring a case founded on the EU “general principles”. These include protections such as the right to equality.
Kate Allen, Amnesty International UK’s director, said: “You don’t have to be pro- or anti-Brexit to see that without reform the EU Withdrawal Bill is set to substantially reduce rights in the UK.
“Under cover of Brexit, the Government is planning to strip the British public of protections - and people don’t even know their hard-won rights are under threat."
The UK must also hold fast to the principles of fair trials, free speech and decent labour standards, she said, adding, “We mustn’t trade away our values in our eagerness to sign new deals.”
During a visit to China earlier in February, commentators said Theresa May had failed to press Chinese Prime Minister Xi Jinping over the treatment of democracy protesters in Hong Kong and instead focus on trade and investment links.
The Prime Minister was praised by Chinese newspaper The Global Times earlier in February for “resisting radical pressure at home”, although she told reporters she had raised the issue.
The report, which assessed the state of human rights in 159 countries, found the scale of attacks on activists around the world was “unprecedented” in the 70 years since the Universal Declaration of Human Rights was adopted.
In the last year, hundreds of activists were killed and journalist were imprisoned across the world. The biggest jailors of journalists were Turkey, Egypt and China, it found.
Amnesty had itself faced threats to its work in Hungary and arrests of its employees working in Turkey, where the organisation’s Chair Taner Kilic is currently jailed.
A spokesperson for the Brexit department said: "The UK has a longstanding tradition of ensuring our rights are protected and our departure from the EU does not change that.
"The Charter of Fundamental Rights was never the source of rights in the UK - it simply reaffirmed rights that already existed in EU law.
"This was reinforced in our published analysis of the Charter which provided guidance about where these rights can be found in UK law and how they will be protected."