LONDON, England (CNN) -- Hand baggage restrictions for passengers traveling from British airports are being eased to try to reduce the chaos caused by increased security checks -- but flights continue to be disrupted.
The British Airports Authority, which operates seven of the country's major airports, said passengers would be allowed to carry one small bag on flights effective Tuesday morning.
CNN's John Vause said that some UK airlines were easing the restrictions from Monday but they would not apply at London Heathrow until Tuesday and disruption there was likely to continue well into the week. (Watch as long delays remain the norm at Heathrow -- 1:14)
The size of bag being allowed as hand baggage was approximately half the size of that previously, he said.
An item of hand luggage must not be more than 45cm long, 35cm wide and 16cm deep including wheels, handles and side pockets, the UK Department of Transport statement said. (Full list of hand luggage restrictions)
Liquids other than prescription medicines and baby milk will continue to be banned, authorities said.
Passengers continued to be asked to consult individual airline Web sites to check on flights.
And the British Airports Authority (BAA) said that passengers arriving Monday should assume the existing restrictions banning hand baggage apart from documents in plastic bags were still in place, as changes would take time to implement.
Thomas Cook Airlines, which fly from 14 UK airports, said it was asking its passengers not to bring hand luggage to airports for the rest of this week.
A spokesman added: "We know some hand luggage is now being allowed on board flights, but we think it will help get flights away with fewer delays if our passengers do not arrive with hand luggage."
The British government downgraded its terror threat level from critical to severe on Monday, saying intelligence suggested that an attack was not imminent. (Full story)
Home Secretary John Reid said the downgraded terror threat level was not a response to airport congestion and flight cancellations.
Some airlines have accused the BAA of being unable to cope with new anti-terror security requirements following the discovery of an alleged plot to blow up airliners mid-Atlantic.
Budget carrier Ryanair appealed to the British government to use police and army reservists to speed up searches at overloaded airport security checkpoints.
Almost a third of flights out of Heathrow were canceled Sunday -- the airport handles about 1,250 flights a day.
A BAA Heathrow spokesman told PA on the easing of luggage restrictions: "This is the beginning on the steps back to normal and we are looking at the best way to implement the changes across our airports."
But he added the changes would take time to implement.
"As of today passengers should not show up with hand luggage. They should continue to operate as they are now operating," he said.
Meanwhile flights continued to be canceled Monday.
Virgin Airways chief executive Steve Ridgway said he was "very unhappy" that it was being asked by BAA to cut the number of flights from Heathrow by a fifth Monday, although the company had reluctantly agreed.
"It's not an ideal situation," said a spokeswoman for British Airways, which has had to cancel 44 Heathrow flights Monday as well as axe 26 London Gatwick services.
Virgin Atlantic Airways' director of communications Paul Charles said: "We are obviously rather frustrated by this. We have concerns about the whole question of resourcing at Heathrow."
He went on: "There is a danger of inconsistency here. On the one hand, the Department for Transport is saying that hand luggage will be allowed on board and then there is BAA saying the ban remains in place at Heathrow today.
BMI chief executive Nigel Turner said its Heathrow flights would also be cut by 20 percent Monday but he hoped the situation would have returned to "pretty much as normal" by the end of the day.
Transport Secretary Douglas Alexander told PA: "I realize the present difficulties at our airports may continue for some time".
These are the new luggage arrangements applying at UK airports:
Each passenger is permitted to carry ONE item of cabin baggage through the airport security search point. The dimensions of this item must not exceed: a maximum length of 45cm, width of 35cm and depth of 16cm.
Other bags, such as handbags, may be carried within the single item of cabin baggage. All items carried by passengers will be X-ray screened.
No liquids of any type are permitted through the airport security search point, other than the following items: Prescription medicines in liquid form sufficient and essential for the flight (e.g. diabetic kit), as long as verified as authentic; baby milk and liquid baby food (the contents of each bottle or jar must be tasted by the accompanying passenger).
To help their progress through search points, passengers are encouraged not to include items capable of containing liquids (e.g. bottles, flasks, tubes, cans, plastic containers etc.) in their cabin baggage.
All laptops and large electrical items (e.g. large hairdryer) must be removed from the bag and placed in a tray so that such items neither obscure nor are obscured by the bag.
Pushchairs and walking aids are permitted but must be X-ray screened. Wheelchairs are permitted but must be thoroughly searched.
In addition to the above, passengers boarding flights to the U.S. and items they are carrying, including those acquired after the central screening point, will be subjected to secondary search at the gate. Any liquids discovered will be removed from the passenger.
The Department for Transport will work closely with operators to introduce these new arrangements, seeking to keep disruption to passengers to a minimum. The department will keep these measures under review.
If passengers have any questions on their travel arrangements or security in place at airports they should contact the airport or their airline.