The Committee against Torture, comprising experts who monitor the implementation of the convention against torture, said the UK should 'refrain from enacting legislation that would grant amnesty or pardon where torture is concerned'.
In its report, the committee expressed concern that no prosecutions for war crimes or torture resulted from investigations by the Iraq Historic Allegations Team - despite there being 3,400 allegations of crimes committed by British forces between 2003 and 2009.
The body also called for an inquiry into alleged acts of torture or ill-treatment of detainees held overseas 'committed by, at the instigation of or with the consent or acquiescence of British officials'.
The report said: 'The committee urges the state party to take all necessary measures to establish responsibility and ensure accountability for any torture and ill-treatment committed by UK personnel in Iraq from 2003 to 2009, specifically by establishing a single, independent, public inquiry to investigate allegations of such conduct.
'The state party should refrain from enacting legislation that would grant amnesty or pardon where torture is concerned. It should also ensure that all victims of such torture and ill-treatment obtain redress.'
The committee also said it was 'seriously concerned' that allegations of torture, ill-treatment, and killing perpetrated in Northern Ireland during the Troubles have not been 'effectively investigated'.
It said the UK should 'refrain from enacting amnesties or statutes of limitations for torture or ill-treatment, which the committee has found to be inconsistent with states parties' obligations under the convention'.
The report comes after defence secretary Penny Mordaunt, Portsmouth North MP, pledged to end the 'chilling' threat of repeated investigations into alleged historical offences by British troops who served in Northern Ireland.
On Wednesday, Ms Mordaunt said she wanted plans to strengthen the legal protections for military personnel who served on overseas operations such as Iraq and Afghanistan to be extended to cover veterans of the Troubles.
The defence secretary has signalled she intends to create a 'statutory presumption' against prosecution of current or former personnel for alleged offences committed in the course of duty abroad more than 10 years ago.
A Ministry of Defence spokeswoman said: 'We are not proposing an amnesty.
'Our package of measures, focused on overseas military operations, will help avoid service personnel and veterans being subject to legal proceedings many years after the events in question where there is no new evidence.
'Wrong-doing in the armed forces will always be investigated in line with our legal and international obligations.'
Sonya Sceats, chief executive at Freedom from Torture, said: 'From Magna Carta onwards, Britain led the way in the evolution of a global ban on torture.
'As (US president Donald) Trump and other torture mongers row backwards, it is time for Britain to lead again by showing we have the courage to confront our shameful support for CIA torture during the so-called war on terror and ensure we never become complicit in torture again.
'At this moment, when the world is wondering what our country stands for, this is an opportunity for the prime minister to show that we still have a moral compass.
'She should answer the UN by ending the evasions and delays and ordering an independent judge-led inquiry into past involvement in torture, as MPs from all parties have demanded.'