David Cameron has condemned the fatal shootings of a rabbi and three children outside a French school as "senseless acts of brutality and cowardice".
The Prime Minister wrote to French president Nicolas Sarkozy to offer his support in the wake of the tragedy, Downing Street has said.
A manhunt is under way after Rabbi Johnathan Sandler, 30, his two young sons and another child were the victims of what was feared to be the latest in a string of racist murders.
The killings at Ozar Hatorah school in Toulouse on Monday were carried out with the same heavy calibre .45 automatic pistol used in two attacks in which three soldiers of North African and Caribbean origin were murdered in the region last week.
Mr Cameron wrote: "I was appalled to learn of the recent shootings that France has suffered, including in Toulouse. People across Britain share the shock and grief that is being felt in France and my thoughts are with the victims, their friends and their families.
"I know that France will draw strength and comfort from your resolute leadership at this difficult time. You can count on my every support in confronting these senseless acts of brutality and cowardice."
Mr Sarkozy flew to the scene of the latest attack and said authorities were raising the terrorism alert to scarlet - the highest on a four-point scale - in Toulouse and neighbouring areas.
"It's a day of national tragedy," he said. "The barbary, the savagery, the cruelty cannot win. Hate cannot win. The nation is much stronger." He announced that 14 riot police units "will secure the region" until the gunman is caught. "We will find him," he added.
Jewish leaders in Britain expressed their shock at the attack and rallied behind the victims' relatives. Chief Rabbi Lord Sacks said he was "deeply shocked and saddened by the atrocious acts of murder".
The killer struck at approximately 8am, just before lessons were due to start, firing 15 rounds at his defenceless targets.