Flight number BA0303 was held on the tarmac at Charles de Gaulle and surrounded by police and fire vehicles at around 8am UK time on Sunday.
British Airways said: "The safety and security of our customers and crew is always our top priority.
"Additional security checks are being carried out as a precaution.
"We would never operate a flight unless it is safe to do so."
A British Airways spokeswoman later confirmed the flight had taken off.
The airline did not state the nature of the alleged threat.
British traveller James Anderson, 20, tweeted a snap of the plane and security services.
He posted: "On British Airways flight BA0303, currently being held on tarmac at Paris due to security threat, surrounded by police and fire vehicles.
"Apparently an individual has made a direct threat to this aircraft."
Anderson later tweeted: "Pilot: 'Aircraft has been deemed absolutely safe. Authorities are satisfied with the search. We will continue on to London Heathrow'."
The security alert comes amid heightened tensions in the UK in the wake of the failed bomb plot on a London Underground train at Parsons Green on Friday.
Intelligence chiefs raised the threat level to "critical" meaning that a second attack could be imminent.
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