The government "cannot guarantee 100% success" in its fight against terrorism, the home secretary says.
Mr Reid said the counter-terrorism effort would be "more agile"
John Reid said he could only promise "100% commitment" from police, security services and the Home Office as it shifts its focus to security.
He said the "struggle" against terrorism would "be unrelenting and of lengthy duration."
He also said terrorists could cause devastation through an "electronic attack" on the UK's infrastructure.
The Home Office is to be split from 9 May, to concentrate on crime reduction, terrorism and mass migration.
The reforms will boost security and provide a "more agile" response to terror threats, Mr Reid told an audience at the Royal United Services Institute.
He said al-Qaeda's aim was to "bleed us to bankruptcy", adding that Western energy supplies could be among targets threatened.
"It is easy to appreciate the devastation of a physical attack and what it can bring but we must not underestimate the potentially devastating consequences of an electronic attack," he said.
The new Office for Security and Counter Terrorism would play a "pivotal role" by enabling the Home Office to focus on personal, community and national security, he added.
Mr Reid said: "It will provide that faster, brighter and more agile response to the terrorist threat through a new drive, cohesion, and by providing a greater strategic capacity to our fight against terrorism."
However, he added: "I can promise you 100% commitment from everyone involved, 100% dedication, but I have to be straight: we cannot promise 100% success.
"That would be an insult to your intelligence, to my integrity, to indicate that we can ever guarantee that in fighting terrorism.
"We are making these changes because we cannot afford one ounce of complacency in this struggle against terrorism."
He praised the work of the police and security services in "heroically defending this nation from terrorist attacks," saying that they had averted "half a dozen" tragedies in recent years.
"The changes that we are introducing by refocusing the Home Office on immigration, crime and counter-terrorism are intended to supplement those efforts," Mr Reid said.
His comments came a day after the UK's counter-terrorism chief condemned as "beneath contempt" people who leak anti-terrorism intelligence.
Deputy Assistant Commissioner Peter Clarke of the Metropolitan Police said leaks had compromised investigations, revealed sources of life-saving intelligence and "put lives at risk" during major investigations.
On Tuesday, Mr Reid told MPs that boosting the Home Office's counter-terrorism effort was expected to cost £15m.
Permanent Secretary Sir David Normington confirmed the department would recruit an extra 150 staff to work in the area.
"This is not just redesignating a minister," Mr Reid told MPs.
"This is adding real capacity, strategic thought, integration, personnel, and resources in the battle against counter terrorism."
The plans to split the Home Office have attracted criticism from some former home secretaries, including Mr Reid's predecessors, Charles Clarke and David Blunkett.