The Prime Minister is in Lancashire as part of the Conservative Party's local election campaign.
84 seats are up for grabs.
Labour consider it one of their biggest targets.
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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.