Cameron joined other world leaders to express his solidarity with the people of Florida following the worst mass shooting incident in the US history.
"I'm horrified by reports of the overnight shooting in Orlando. My thoughts are with the victims and their families," Cameron tweeted.
Armed with an assault rifle and a handgun, the attacker Omar Mateen opened fire on revellers in the packed gay nightclub yesterday morning.
The FBI said the 29-year-old, who was known to US authorities, legally purchased two firearms within the last week.