Former detainee Mr Mohamed alleges that he was tortured into falsely confessing to terrorist activities and claims MI5 officers were complicit in his abuse.
In his statement, Mr Mohamed said: "I have to say, more in sadness than in anger, that many have been complicit in my own horrors over the past seven years.
"For myself, the very worst moment came when I realised in Morocco that the people who were torturing me were receiving questions and materials from British intelligence."
Mr Mohamed, a British resident, aged 30, had been held at Guantanamo Bay for more than four years.
An aircraft carrying the 30-year-old landed at RAF Northolt in north west London just after 1pm on Monday.
Concerns had been expressed about Mr Mohamed's health after he recently went on hunger strike, but he was able to walk off the aircraft without needing support.
He was dressed in casual clothes and clutched what appeared to be a document holder as he made his way to the terminal building surrounded by officials.
Ethiopian-born Mr Mohamed lived in London before his arrest in Pakistan in 2002.
He was held at the controversial US military detention centre at Guantanamo Bay on Cuba from September 2004.
The British Government requested his release, and a team of British officials went to Guantanamo Bay recently to check that he was well enough to travel back to the UK.
Foreign Secretary David Miliband said he was pleased the former detainee had returned to the UK.
He said: "This is the direct result of our request for his release and return, and follows intensive negotiations with the US government.
"Mr Mohamed was accompanied by Foreign and Commonwealth Office officials, officers of the Metropolitan Police and the doctor who visited him at Guantanamo Bay last week.
"We have been in touch with Mr Mohamed's family and legal representatives to inform them of his return.
"The UK Government requested the release and return of all former legal UK residents detained at Guantanamo Bay in August 2007.
"In reaching this decision we have paid full consideration to the need to maintain national security and the Government's overriding responsibilities in this regard."
Mr Mohamed will spend the next few days with his family and legal team.
In his statement, he said: "I hope you will understand that, after everything I have been through, I am neither physically nor mentally capable of facing the media on the moment of my arrival back to Britain.
"I have been through an experience that I never thought to encounter in my darkest nightmares.
"Before this ordeal, torture was an abstract word to me. I could never have imagined that I would be its victim.
"It is still difficult for me to believe that I was abducted, hauled from one country to the next, and tortured in medieval ways - all orchestrated by the United States government.
"While I want to recover, and put it all as far in the past as I can, I also know I have an obligation to the people who still remain in those torture chambers.
"My own despair was greatest when I thought that everyone had abandoned me. I have a duty to make sure that nobody else is forgotten."
Speaking about his period of torture in Morocco, Mr Mohamed said: "I have met with British intelligence in Pakistan. I had been open with them. Yet the very people who I had hoped would come to my rescue, I later realised, had allied themselves with my abusers."
He went on: "I am not asking for vengeance, only that the truth should be made known so that nobody in the future should have to endure what I have endured."
Kate Allen, director of Amnesty International UK, called for an independent inquiry into Britain's role in secret detention and rendition programmes.
"We're pleased that we have Binyam back today but it has taken the British Government far too long to be arguing for these men," she said.
Mr Mohamed's UK lawyer, Clive Stafford Smith, said he was "absolutely" convinced of the former detainee's innocence.
"If anyone wants to put him on trial, in the immortal words of George Bush, bring them on," he said.
Gordon Brown declined to say whether Mr Mohamed will face any restrictions on his liberty when he returns to the UK.
Speaking in Southampton, where he is attending a Cabinet "awayday", the Prime Minister said: "What I can say is that at all times the security of the country will be protected.
"Of course, we have got to look at the details of the arrangements, but at all times the security of the British people comes first."
He added: "My first concern is the security of people in this country and we will do everything in our power to protect the security of people in our country and the Home Secretary will take whatever action is necessary."
Mr Mohamed was detained under Port and Border Controls, part of the Terrorism Act 2000, when he arrived in the UK, Scotland Yard said.
A spokesman said he was not arrested.
He added: "The man has been monitored by a forensic medical examiner during the flight to ensure that he is fit to be detained by police.
"He will have access to a solicitor, as is normal practice.
"Police are conducting investigations into his case. Their inquiries are being carried out, as they must be, strictly in accordance with UK law."