British Prime Minister David Cameron Monday proposed expansion of police powers and a deradicalization program to head off terror plots hatched by returning militants. An estimated 500 Britons are suspected of fighting alongside Islamic State militants, the Guardian reported.

The Conservative Party’s Cameron proposed seizing the passports of suspected militants and forcing terror suspects into deradicalization programs to reverse their fanaticism. The Telegraph reported suspects also could be forced to move from their hometowns. In addition, airlines would be required to provide more information about passengers.

Addressing the House of Commons, Cameron called the idea of British citizens swearing allegiance to militant groups such as the Islamic State, formerly known as ISIS, “abhorrent.” He said he is looking for ways to keep them from returning to the U.K.

“Adhering to British values is not an option or a choice,” Cameron said. “It is a duty for all those who live in these islands, so we will stand up for our values, we will in the end defeat this extremism, and we will secure our way of life for generations to come.”

Cameron said he has been “shocked and sickened by the barbarism” exhibited by Islamic State militants as they seized territory in Syria and Iraq, including the “widespread slaughter” of fellow Muslims and the persecution of religious minorities.

The Guardian called Cameron’s proposals “curiously understated” and noted passports “are not an automatic right. The Home Office can take them away.”

“When police suspect a traveler at the border, they are not currently able to apply for the royal prerogative [to revoke a passport] and so only have limited stop-and-search powers,” Cameron said. “We will introduce specific and targeted legislation to fill this gap by providing the police with a temporary power to seize a passport at the border, during which time they will be able to investigate the individual concerned. This power will include appropriate safeguards and oversight arrangements.”

However, criminalizing travel to certain countries would be “wrong,” he said.

“We have made clear we want to work with the government on evidence-based plans to stop people being drawn into extremism and deal with the threat,” Yvette Cooper, the shadow home secretary of the opposition Labor Party, said in a statement issued in response to Cameron. “We need both strong powers and strong safeguards to protect democracy and keep our country safe.”

Cameron’s proposals did not universal support, however. Reuters reported Dominic Grieve, the government’s former top lawyer, said he was concerned about preventing British citizens from returning home. “Not only does it offend principles of international law, it actually would offend basic principles of our own common law as well,” Grieve said.

Cameron’s remarks follow an increase in the terror threat level to “severe,” which means an attack is “highly likely.”