As part of his final acts before leaving office, outgoing US President Barack Obama has shortened the prison sentence of over 200 inmates, including Chelsea Manning, the former military intelligence analyst responsible for the biggest breach of classified materials in US history.

Obama on Tuesday reduced Manning’s sentence from 35 years to seven years. Manning was one of 209 inmates whose sentences Obama was shortening, White House Counsel Neil Eggleston said in a statement.

Eggleston said “Today, 209 commutation recipients – including 109 individuals who had believed they would live out their remaining days in prison – learned that they will be rejoining their families and loved ones.”

The 28-year-old transgender whistle-blower, who changed her name from Bradley to Chelsea after her sentencing, will be released in May.

She has been imprisoned since 2010 and was sentenced in 2014 to 35 years in prison for espionage, by far the longest punishment ever imposed in the United States for a leak conviction.

Manning was working as an intelligence analyst in Baghdad in 2010 when she provided more than 700,000 documents, videos, diplomatic cables and battlefield accounts to anti-secrecy group WikiLeaks, the largest leak of classified materials in US history.

Court testimony showed that she had been in a mental and emotional crisis before the leaks, due to the stress of a war zone and the fact that she had gender dyphoria.

Manning has already served nearly seven years in prison. She tried to kill herself on two separate occasions while in prison last year.

However, Obama’s commute angered some Republican lawmakers.

US House of Representatives Speaker Paul Ryan in a statement said the decision was a "dangerous precedent" for those who leak materials about national security.

Ryan said "This is just outrageous. Chelsea Manning';s treachery put American lives at risk and exposed some of our nation';s most sensitive secrets."

Republican Senator Tom Cotton said the leak endangered troops, intelligence officers, diplomats and allies. "We ought not treat a traitor like a martyr," Cotton said.