The director of the FBI has conceded that future judges will look to his battle with Apple as a precedent for law enforcement access to locked or encrypted mobile devices, the first time the government has conceded that the implications of the case stretch beyond an investigation into the San Bernardino terrorist attacks.

The ultimate outcome of the Apple-FBI showdown is likely to “guide how other courts handle similar requests”, James Comey told a congressional intelligence panel on Thursday, a softening of his flat insistence on Sunday that the FBI was not attempting to “set a precedent”.

Comey deferred answering legislators’ questions on the implications of a judge ordering Apple to write code permitting the FBI to guess the passcode of an iPhone 5C used by the San Bernardino killer Syed Farook, something Apple has painted as sufficiently wide-ranging to justify its defiance of a court order.

The director repeatedly said he was “not an expert”, but that technical and legal experts had advised him that “technology [is] the limiting principle”, because the particular combination of the outmoded iPhone 5C and its iOS9 mobile operating system limited the application of the requested court order – a claim Apple rejects.

While Comey continued to argue that the FBI needed to seek access to data on the iPhone for its terrorism investigation of San Bernardino, he acknowledged that police departments and district attorneys around the country were also seeking similar access to locked phones and encrypted conversations in ordinary criminal cases.

Manhattan prosecutor Cyrus Vance has said he has a backlog of 175 locked iPhones awaiting the resolution of the Apple-FBI fight, which is almost certain to be decided in high federal courts.

The outcome “will be instructive for other courts”, Comey told the House intelligence committee.

“I don’t know how lawyers and judges will think about what is the limiting principle on the legal side.”

Apple on Thursday morning challenged Comey’s suggestion that the passcode-breaking tool described in the order could be limited in scope to a single iPhone.

The company referred back to CEO Tim Cook’s statements that the underlying code Apple has been ordered to create is highly transferrable to other phones, to the extent that a similar password-cracking program for another device would be virtually the same.

Technologist and Apple operating system expert Jonathan Zdziarski wrote last week that the basics of the tool Apple has been instructed to build require that it work on multiple phones simply to ensure that it works at all. A senior government official told ABC News on Wednesday that many police departments were anxious to exercise the same privilege.

Cook has also said he is troubled by the precedent set should the high court uphold the order. Comey, testifying on Thursday to the House panel, acknowledged that the outcome of the case would be “instructive to other courts”.

Also for the first time since the 16 February order sent shockwaves around the technology, law enforcement and cybersecurity worlds, Comey softened some of the government’s harsh rhetoric around Apple.

“There are no demons here,” said Comey, striking a more conciliatory tone than that of the Justice Department’s accusation in court last week that Apple was placing “marketing” over security.

Comey’s tone is not shared throughout the US Department of Justice. The attorney general, Loretta Lynch, told the congressional appropriations committees this week that within the department’s $781m budget request is an initiative devoting $38m “toward developing the tools we need to lawfully access encrypted data and communications”.

Lynch listed secure communications alongside serious criminal threats, saying that the Justice Department needed “critical measures to address evolving challenges like homegrown extremism, online radicalization and increasingly sophisticated encryption”.

Apple is expected to file its first formal legal response explaining its resistance to the access order by Friday. In advance, both sides have waged a high-profile messaging war.

Cook, in an interview with ABC on Wednesday, said the FBI was asking Apple to write code – which engineers have derisively termed “FBiOS” – that would serve as “the software equivalent of cancer”, opening the door not only for future forced access to data on a phone, but to remote law enforcement hijacking of its functions.

“Maybe it’s an operating system for surveillance, maybe the ability for the law enforcement to turn on the camera. I don’t know where it stops,” Cook told ABC.

Jim Himes, a Connecticut Democrat on the panel, suggested that once Apple writes the code the FBI desires, the company would become a target of “our sovereign adversaries, of criminal enterprises, of terrorists” attacking the Apple servers on which the company stores the tool – implying that the FBI would in effect mandate the creation of a cybersecurity vulnerability for Apple’s millions of mobile customers.

“There’s a legitimate worry, though, that a decision in favor of the FBI could be the narrow end of a very wide wedge,” Himes said, reframing the debate as “security versus security”.

Comey said he lacked the engineering or legal expertise to address the “reasonable questions” in full, but contradicted software engineers to say the worry of the code getting into other hands was “not a real thing” and praised Apple’s track record on security.

Apple has “done a pretty darn good job of protecting its code”, Comey said.

The House intelligence panel was the first to hear from Comey since the 16 February warrant to Apple, although the committee hearing was not specifically focused on the Apple case. Next week, the House judiciary committee is scheduled to take up the controversy, though it is not yet clear who will testify.

Some Republicans on the panel signalled their early inclination to back the FBI over Apple. Lynn Westmoreland, a Georgia Republican, said the case looked “no different than what you do with a [foreign intelligence] or any other warrant”.