Security services have prevented seven attacks in the UK over the last six months, David Cameron said as he warned a Paris-style attack "could happen here".
The terror spree in France "was the sort of thing we warned about" in planning with the security services but such atrocities meant "you have to go right back to the drawing board" to work out what more steps needed to be taken, the Prime Minister said.
Asked if Britain was at war with the Islamic State terror group, which has claimed responsibility for the slaughter, Mr Cameron told BBC Radio 4's Today programme that the UK stands in "total solidarity" with France and would do "everything we can" to defeat the jihadis.
The Prime Minister was speaking shortly before a meeting with Vladimir Putin on the fringes of the G20 summit in Turkey, at which he urged the Russian president to "work together" with Western powers in the international struggle against IS, which Britain believes is also responsible for downing a Russian airliner in Egypt last month.
Mr Cameron offered Mr Putin his condolences for the 224 deaths on board the Metrojet flight from Sharm el-Sheikh and told him: "We are meeting together after the appalling terrorist attacks in France, and it is clear to me that we must work together to defeat this scourge of terrorism that is a threat to Britain, a threat to Russia and a threat to us all."
The Russian president has also spoken to US President Barack Obama, German Chancellor Angela Merkel and Italian PM Matteo Renzi within the last 24 hours as Western leaders combined to make overtures to Russia to co-operate over Syria.
Mr Putin told Mr Cameron that UK-Russian relations were "not in the best shape", but thanked him for sharing UK intelligence in a phone call following the Sharm crash and added: "The recent tragic events in France show that we should join efforts in preventing terror."