The US authorities released Saudi national Shaker Aamer from Guantanamo Bay after he spent 13 years without trial in the military prison. Aamer is married to a British woman and has four children.
The British resident was on his way to Europe, UK officials announced on Friday.
Andy Worthington from "We Stand With Shaker" campaign confirmed the news of Aamer's release.
"We're delighted to hear that his long and unacceptable ordeal has come to an end," he said.
"We hope he won't be detained by the British authorities on his return and gets the psychological and medical care that he needs to be able to resume his life with his family in London," he added.
Terrorism versus charity
Aamer was arrested in Afghanistan in 2001 and handed to the US military, which transferred him to Guantanamo in 2002, according to the rights group Reprieve.
The US authorities suspected the 46-year old Shaker Aamer of acting as a recruiting sergeant, financier and fighter for al Qaeda, as well as cooperating closely with Osama Bin Laden.
According to Reprieve and Aamer himself, the Saudi national was doing volunteer work for an Islamic charity in Afghanistan.
Despite the allegations of terrorism, Aamer was never formally charged nor put on trial.
Pushing to close Guantanamo
The US officials cleared Aamer for release in 2007, and again in 2009, but he was not released from the controversial facility within the US military base in Cuba.
The decision to free Aamer comes after years of lobbying by the UK officials, including a personal appeal from the UK Prime Minister David Cameron to the US President Barack Obama.
Aamer is a father of four, is married to a British woman and holds an unlimited residence permit in the UK.
He is the last British resident to be freed from Guantanamo as part of a drawn-out effort to close the infamous prison.
dj/kms (AFP, Reuters, AP, dpa)