Between 750 and 800 Britons have gone to Syria and Iraq, and 70 have been killed, the Home Office's counter-terrorism chief has told MPs.
Charles Farr said about half had since returned to the UK.
MPs on the Commons Home Affairs Select Committee also heard from the UK's top counter-terrorism officer who repeated warnings about police budget cuts.
Met Assistant Commissioner Mark Rowley said "dramatic" cuts would hamper the police's ability to tackle terrorism.
Mr Farr and Mr Rowley were giving evidence on the eve of the government's Spending Review, which sets the budgets of all the government departments.
MPs were told the police's ability to respond to an attack by gunmen armed with automatic firearms had been reviewed last week following the attacks in Paris.
Mr Farr, director of the Office for Security and Counter-Terrorism, dismissed claims that police did not have the right kind of guns to respond to an attack with automatic weapons, saying he was "completely satisfied" with the "variety" of firearms available.
During the session, Mr Rowley confirmed that a letter leaked last week about the impact of police cuts was from him.
Mr Rowley said he was "very disappointed" his letter to the home secretary had been made public, but he remained concerned that if "dramatic" cuts were imposed they would undermine the police's ability to gather intelligence and respond to a major terrorist incident.
Chancellor George Osborne has said counter-terrorism funding will rise by 30%, but it is believed he could announce cuts to the police budget of more than 20% when he addresses Parliament on Wednesday.
Mr Rowley told MPs: "In terms of the relevance of policing budgets, whilst we have this ring-fenced budget for counter-terrorism, it's not a separate machine, it depends very heavily on local policing resources."
He told the committee that a 20% cut would "certainly" pass the tipping point of what could be absorbed.
He also told MPs that British police were aiding the hunt for the plotters behind the Paris atrocities and were attempting to "harvest any information" about the attackers that may be relevant to the UK.
The so-called Islamic State militant group has said it carried out the wave of shootings and suicide bombings in Paris earlier this month, which killed at least 130 people.
Most of the Britons who have travelled to the conflict zone in Syria and Iraq are thought to have joined IS, which came to international attention in 2014 when it seized large swathes of territory.
The BBC has a database which tracks British jihadists - the information has been compiled from open sources and BBC research.