The government does not yet have enough votes from MPs to back air strikes against ISIL terrorists in Syria, Britain's Defence Secretary Michael Fallon said on Sunday.
He told the BBC that he had been in talks with Labour MPs but "we've got to keep building the case".
Fallon admitted it would be harder to get the support if Labour ordered its MPs to vote against air strikes in Syria.
Labour's Jeremy Corbyn said he opposed the strikes but he had not decided whether to give his MPs a free vote.
Fallon told the BBC that it was dangerous not to try to defeat the ISIL with force.
"We also have to defend ourselves... we are already a top target," he said.
"There are always risks in war but there is a greater risk from not doing something about ISIL and leaving our streets vulnerable to the kind of slaughter we saw in Paris."
Britain has so far refrained from bombing ISIL in Syria after MPs rejected a vote for air strikes against Syrian government targets in 2013. However, it is taking part in air strikes against the same group of terrorists in Iraq.
Fallon admitted there were legitimate questions to answer but added: "You could see opinion beginning to shift."
Corbyn said there were strong views on both sides within his party, but he said that Labour members who he believes are against bombing "must have a voice".
"We will come to a decision as a party," he said.
Corbyn said he also feared that bombing Syria could increase the number of people becoming radicalised.