In his speech to business leaders, David Cameron is expected to acknowledge that we're living in "perilous economic times".
"Turn on the TV news and you see the return of a crisis that never really went away: Greece on the brink…the survival of the Euro in question," he is expected to add.
"Faced with this, I have a clear task: to keep Britain safe.
"Not to take the easy course - but the right course."
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
In her message Theresa May praised the "inspirational bravery" and "unbreakable spirit" of Mancunians in the wake of the blast.