David Cameron said we are in "a battle for Britain's future" ahead of this weekend's G8 summit.
The Prime Minister said the UK must not "hide away from the world" but instead "roll up our sleeves and compete in it".
"It's no use in giving into the world. We must be unashamedly bold and hard-headed about pursuing our national interests", he continued.
"The challenge is clear, we are in a battle for Britain's future".
More top news
Teenagers, aged 13 and 14, were taken to hospital on Friday morning but passed away, South Yorkshire Police said.
Eleven people remain in hospital.
The 22-year-old wore the full uniform, including hairnet, in Middlesbrough.