Airports in north-west England are on a high-security alert after an operation to foil an alleged plot to blow up planes from the UK in mid-flight.
Queues began forming early at Manchester airport
Passengers at Manchester Airport are facing delays of up to 90 minutes, with domestic flights experiencing more disruption than international ones.
Delays have also been reported at Liverpool John Lennon Airport, where some flights have been cancelled.
Passengers travelling from Blackpool Airport are asked to check in early.
Police patrols have been stepped up at all airports in the region.
To help reduce disruption and anxiety, staff at Manchester Airport have been handing out security notices explaining the delays and added security measures.
They have also been distributing plastic bags for passengers to carry permitted hand luggage on board.
Police sniffer dogs and extra officers, including armed police, are patrolling the airport.
Armed police are on patrol for passenger safety
A spokesman for the airport said: "I think the message to travellers is come to the airport as normal.
"We are doing everything we possibly can to minimise disruption and make sure they get on their flights as securely and safely as possible.
"There is additional security staff helping all the passengers get through the airport as quickly as possible.
"People can expect to have searches at security including searches of their footwear."
Virgin Trains said it was accepting passengers with airline tickets between Manchester and London.
Travellers with valid tickets for BA and BMI to Heathrow, BA and Jet2 to Gatwick, VLM to London City and Air Berlin to Stansted are welcome to travel by train, the company said.
In Liverpool, flights to a number of destinations have been cancelled - including Belfast, Berlin, Carcassonne and Venice - and strict security measures have been imposed.
Police patrols have been stepped up in the terminal building, pick up and drop off points cordoned off and only passengers are allowed entry to the terminal building.
All terminal pick up and drop off points have been cordoned off
A spokesman said: "There was a lot of congestion and queues first thing this morning but all the passengers were very cooperative as they understand why we need to put these measures in place.
"The operation is running well at the moment and we will work hard to minimise disruption as much as possible."
In Lancashire, Blackpool Airport has also increased security measures and staff are advising passengers to give themselves extra time to check in for flights.
A spokeswoman said: "We are asking people to be careful with their baggage - they won't be allowed certain items.
"There will be additional security through baggage and when people are searched, so we do anticipate security checks will take slightly longer than normal."
Lancashire Constabulary has increased the number of officers on duty to reassure travellers, the force said.
Assistant Chief Constable Mike Cunningham urged passengers not to be alarmed by the increased police presence.