In a government statement, MP Theresa May said: "The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West.
"Some of those plots are likely to involve foreign fighters who have travelled there from the UK and Europe to take part in those conflicts.
"The first and most important duty of government is the protection of the British people."
The raised threat level comes following the beheading of American journalist James Foley, who was believed to have been killed by a British national.
They also revealed that they would be increasing patrols to help people feel safe in the wake of the higher threat level.
VICTIM OF FORCED MARRIAGE REVEALS SHE WAS RAPED ON HER WEDDING NIGHT
A government spokesperson said: "From this afternoon we will begin to increase our levels of visible patrols and implement other security and protection measures.
"We will also build on existing community relations to provide reassurance and seek their support and assistance in keeping the UK safe."
Prime Minister David Cameron has also unveiled plans to seize the passports of British citizens planning to travel abroad and fight in conflict zones such as Syria and Iraq.
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