Conservatives are among MPs battling to stop the government granting immunity to British ministers, spies and military personnel for involvement in murder and torture abroad.
Tory grandee David Davis is bringing a series of amendments to the National Security Bill, which has alarmed human rights groups and anti-war crime campaigners.
The proposed changes, aiming to strip out clauses granting legal immunity and restricting damages for torture survivors, will be debated in the House of Commons later on Wednesday.
In a move backed by the former justice secretary Robert Buckland, opposition MPs are also attempting to add a public interest defence into the Official Secrets Act to protect whistleblowers and journalists.
In a letter to the prime minister, a group of charities including Reprieve, Liberty, Amnesty International UK and Freedom From Torture questioned why the government would “seek to protect themselves and their officials” from prosecution for assisting or encouraging crimes abroad.
“It is unconscionable that the British government might shield ministers or officials from accountability,” the letter said.
“With no limits on what criminal activity the clause includes, this provision could be used to shield British ministers and officials from prosecution where they have encouraged or assisted extraordinary rendition, interrogations involving torture, or unlawful targeted killings.”