The chairman of the Senate intelligence committee, Dianne Feinstein, said on Thursday she believed a court order compelling Verizon to hand over call data relating to millions of Americans had been in place since 2006.

At an impromptu press conference on Capitol Hill, in the wake of the Guardian's revelations about the court order, Feinstein said it was required to keep Americans safe. "This is called protecting America," said Feinstein. "People want the homeland kept safe."

Feinstein said she believed the order had been in place for some time. She said: "As far as I know, this is the exact three-month renewal of what has been the case for the past seven years. This renewal is carried out by the [foreign intelligence surveillance] court under the business records section of the Patriot Act. Therefore, it is lawful. It has been briefed to Congress."

Asked by the Guardian why it was necessary for the FBI to have such sweeping powers, Feinstein said it was so that they had access to the phone numbers in case they became terrorist suspects in future.

Late last year, Feinstein shepherded a Senate reauthorisation of a law vastly expanding government surveillance activity inside the United States. With the support of the Obama administration, Feinstein helped quash amendments to the 2008 update to the Foreign Intelligence Surveillance Act that would have required the National Security Agency to, among other things, estimate how many Americans' communications it intercepts, something the NSA has declined to specify.

"Citizens generally assume our government is not spying on them," Senator Jeff Merkley, Democrat of Oregon, told US News during the December debate. "If they had any inkling of how this system really works, the details of which I cannot discuss, they would be profoundly appalled."

Feinstein went on to say she does not know how metadata of the kind collected under the Verizon court order is used. The metadata includes phone numbers, time and location of calls, call duration and other data.

Senator Saxby Chambliss, the ranking Republican on the committee, speaking alongside Feinstein, added that the FBI were mainly interested in who might be on the end of the phones calls made from America to overseas terrorist suspects. "Senator Feinstein tells me her committee authorised sweeping FBI powers in case they needed telephone numbers in the future."

'This has been going on for seven years'

Chambliss said it was "nothing new" and the material supplied was "simply" metadata.

However, in 2013, such metadata can provide authorities with vast knowledge about a caller's identity. Particularly when cross-checked against other public records, the metadata can reveal someone's name, address, driver's licence, credit history, social security number and more. Government analysts would be able to work out whether the relationship between two people was ongoing, occasional or a one-off.

"This has been going on for seven years," Chambliss said. "Every member of the United States Senate has been advised of this. To my knowledge there has not been any citizen who has registered a complaint. It has proved meritorious because we have collected significant information on bad guys, but only on bad guys, over the years."