Labor offers single debate
The prime minister says the debate was planned before Fri포커day's terror attacks in Paris.
Labour said the government was "flirting" with a possible ban on foreign fighters joining the forces.
The issue was raised by leader Ed Miliband at the party's launch, in an interview with Radio 5Live's Today programme.
"This issue was on the table and we had an opportunity to have a debate. We couldn't because there was still an atmosphere of terror, violence and death that had been caused by people who were going around on UK soil."
On Friday, French police said three of the attackers in the attacks had joined Islamic State militants.
Earlier this year, Labour and the Lib Dems raised the possibility of banning any foreign fighter from joining the military at the first chance, saying some British soldiers may be fighting with extremist groups.