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Bomb disposal teams have been sent to the Port of Dover and a ferry has been evacuated after a suspicious package was found on board.
Passengers were led off the 6.40am Calais service and emergency services rushed to the scene.
P&O Ferries warned there will be delays to later services through the morning. At 10.15am a spokesman said the incident was over. The army has now left, but the exact nature of the suspicious package has not been revealed.
Highways England said long delays remained at the port for most of the morning.
Staff and crew could be seen standing outside the port's food hall and some cars had to be left on board the vessel.
A bomb disposal van from the Royal Logistic Corps was parked at the dockside as were the Port of Dover Police and a Incident Response Unit from the South East Coast Ambulance Service.
A P&O Ferries spokesman said: “A suspicious package was found on one of our ships this morning, the authorities are currently in attendance, and all passengers and non-essential crew have been evacuated as a precaution.”
They added all passengers affected would be able to leave on the next available sailing.
A Ministry of Defence spokesman said troops from Shorncliffe Barracks, near Folkestone, were at the docks.
They added: "The call came through around 6.30am. At the moment, we do not know the exact nature of the incident until it has been concluded."
In a message to passengers, P&O Ferries said: "We're sorry to say that due to a security incident, we are expecting possible delays while police are in attendance. Thank you for your understanding."
A Kent Police spokeswoman said the force was called just before 7am, adding: "We are attending the Port of Dover to assist Port of Dover Police with an incident on a ferry." The 6.40am crossing has been cancelled, P&O said.
The company added future sailings will see delays of up to 75 minutes.
It said the next departures will be the 8.40am sailing, expected to leave around 9.30am and the 10.45 departure is not due to depart until midday.
The tweet added: "Please check in as normal, we'll get you on your way asap."
Highways England is warning motorists heading to the port of potential delays - we will have details of travel disruption on kmfm.
The incident comes just one day after bomb disposal teams were called to Kent to deal with an unexploded Second World War bomb on a site being turned into housing.
Among those caught up in the chaos were Steph and John Gill from Minster on the Isle of Sheppey.
Mum-of-two Steph said: "We have had a three-hour delay but are now on board. We are heading for Boulogne then City Europe."
Husband John added: "We will get to France eventually but we will have to cut our trip short. We were originally hoping to visit Bruges in Belgium but we are determined to still have a fun family day together."