LONDON » Britain’s domestic intelligence chief has demanded greater authority for spies to help fight the threat of Islamist extremism, a sign that the attack on a satirical newspaper in Paris is likely to sharpen the security-versus-privacy debate in Western countries.

Andrew Parker, the director general of Britain’s Security Service, known as MI5, said militants were planning attacks in Britain similar to the one that killed 12 people at the newspaper, Charlie Hebdo.

More than 600 Britons have traveled to Syria to join jihadists, he said, and three terrorist plots in Britain have been stopped by security services in recent months alone.

"Death would certainly have resulted otherwise," he said.

Amid a backlash against digital surveillance after disclosures by the former National Security Agency contractor Edward J. Snowden in 2013, Parker said there was a growing imbalance between the number of terrorist plots against Britain and the ability of spies to track their communications.

Speaking at MI5 headquarters Thursday, he warned against an atmosphere in which privacy was "so absolute and sacrosanct that terrorists and others who mean us harm can confidently operate from behind those walls without fear of detection."

"If we are to do our job, MI5 will continue to need to be able to penetrate their communications as we have always done," he said. "That means having the right tools, legal powers and the assistance of companies which hold relevant data."

"Currently," he added, "this picture is patchy."

Parker also offered perhaps the starkest assessment yet of the homegrown Islamist threat, citing more than 20 Syrian-linked terrorist plots against Western targets over the past 14 months.

In recent months, intelligence services in Britain and the United States have been campaigning publicly against pressure to rein in their surveillance operations, notably pitting them against the U.S. technology companies that dominate the Internet, like Google, Facebook and Apple.

Robert Hannigan, the recently appointed director of GCHQ, Britain’s electronic intelligence agency, castigated Internet companies in November for providing the "command-and-control networks of choice for terrorists and criminals" and challenged them to find a better balance between privacy and security.

Companies are stepping up efforts to strengthen encryption, saying they are responding to demands for more privacy from their users.

Governments and their security services, technology executives have argued, should obtain a warrant before gaining access to private communications.

The Paris attacks, however, hold the potential to shift the debate.

Reacting to Parker’s speech, held before an invited audience, the chancellor of the Exchequer, George Osborne, said the government would give intelligence services what they needed.

"My commitment is very clear," Osborne told the BBC. "This is the national priority. We will put the resources in. Whatever the security services want, they will get."

Already, he said, the government is spending more than 100 million pounds ($151 million) to monitor Britons traveling to conflicts in Syria and Iraq.

Elsewhere, officials also appeared to call for measures that make counterterrorism a higher priority than certain civil liberties. The president of the European Council, Donald Tusk, said Friday that the Paris attack highlighted the need for airlines to share passenger data, in order to better track extremists traveling to and from Europe and conflict zones.

A proposal that would grant the security services access to several years of travel data in and out of the European Union was blocked by the Civil Liberties Committee of the European Parliament because of privacy concerns. But Tusk urged lawmakers to fast-track the proposal.

Europe’s response to the Paris attack will top the agenda during a Feb. 12 summit meeting. "The EU cannot do everything," Tusk said, but it "can contribute on strengthening security."

Parker described the attack on the newspaper as "a terrible reminder of the intentions of those who wish us harm."

About half of MI5’s investigations focus on counterterrorism, and almost all of them intercept communications in one way or another to identify plotters and plots, Parker said.

"If we lose that ability, if parts of the radar go dark and terrorists are confident that they are beyond the reach of MI5 and GCHQ, acting with proper legal warrant, then our ability to keep the country safe is also reduced," he said.

"My sharpest concern as director general of MI5," he said, "is the growing gap between the increasingly challenging threat and the decreasing availability of capabilities to address it."

Although he said that the threat level in Britain had worsened, the country had not raised its terror alert level beyond its current level, the second highest.

Extra security staff members have been posted at critical hubs like airports and train stations, as well as at border points with France.

An attack in Britain was "highly likely," he said, adding that MI5 could not guarantee it would be able to stop it.

"Although we and our partners try our utmost, we know that we cannot hope to stop everything," he said.

Katrin Bennhold, New York Times