Passengers should be allowed to carry more than one bag on to a plane from January as security restrictions at UK airports are eased, Gordon Brown says.
Security will be stepped up at railway stations, airports and ports as part of government attempts to tackle terrorism, Gordon Brown has announced.
He also outlined measures to give better protection against terror attacks on rail stations, airports, shopping centres and sports stadiums.
There will be new security barriers, vehicle exclusion zones and blast resistant buildings, but air passengers will be allowed more luggage from 2008.
Security minister Lord West was asked to draw up the plans in July.
Rail travellers at large stations will also face having their bags screened.
However, ahead of the announcement Lord West was at the centre of confusion on detention limits for terror suspects.
The PM's statement came amid confusion over his security minister's views on detention limits for terror suspects.
He told the BBC initially he was not convinced about extending the limit beyond 28 days - but then said he was convinced, after talks in No 10.
During the Today programme interview at 0820, former Admiral Lord West said he still needed "to be fully convinced that we absolutely need more than 28 days".
In his wide-ranging Commons statement on national security, Mr Brown said that the failed bomb attacks in London and Glasgow Airport in June showed that Britain needed to ensure young people are not "radicalised" by extremists, in addition to raising military and security counter-terrorism measures.
'Complex plots'
He outlined the creation of a new unit bringing together police and security intelligence to look not only at the "inner circle" of extremists but also at those at risk of falling under their influence.
"I want to be totally convinced because I am not going to go and push for something that actually affects the liberty of the individual unless there is a real necessity for it."
He said no "major failures in our protective security" had been uncovered in a review led by Lord West.
But then at 0930, after a half-hour meeting with Mr Brown, the peer told the BBC he was "personally convinced" that the 28-day limit needed extending.
Cabin baggage
There is no doubt there are very real threats, that the design in the past of certain buildings and certain places does not make it easy to counter a terrorist attack on them Lord WestSecurity Minister
But he said improved security would be installed at the country's 250 busiest railway stations, as well as airports, ports and more than 100 other sensitive locations.
"I personally, absolutely believe that within the next two or three years we will require more than that for one of those complex plots," he said.
Guidance would be sent to thousands of cinemas, theatres, restaurants, hotels, sporting venues and commercial centres, as well as all hospitals, schools and places of worship to advise them on how to keep visitors safe against terrorism.
He later issued a statement in an attempt to clarify his position saying: "I am quite clear that the greater complexities of terrorist plots will mean that we will need the power to detain certain individuals for more than 28 days."
Downing Street said Mr Brown had full confidence in the peer and denied suggestions the prime minister had influenced Lord West's second statement.
Companies responsible for crowded places would be given updated advice on how they can improve resilience against attack.
The minister later stressed that he had not changed his position. "Maybe being a simple sailor and not a politician, I didn't choose my words very well," he said.
Around 160 counter-terrorism advisers will train civilian staff to watch out for suspect activity, ensure premises have adequate emergency facilities and make best use of their CCTV footage.
Tories, Lib Dems and some Labour MPs plan to try to block the move to raise the limit, saying there is no evidence change is needed.
But Mr Brown said from 8 January airports would be able to seek permission to let passengers take more than one item of hand luggage to be taken on board flights by passengers.
Lord West's comments came as Mr Brown outlined conclusions from the peer's review into strengthening security in crowded public areas and designing anti-terror features in new buildings and projects.
However, size restrictions on liquids and cabin luggage would remain.
Mr Brown was also unveiling more details of the planned border force combining immigration, customs and visa staff and a relaxation of air luggage rules.
The prime minister also outlined measures to counter the influence of radical fundamentalists in Britain's schools, universities, mosques, youth clubs and prisons, as well as on the internet.
Ahead of the statement he said that there would be "some inconvenience in what we propose", but this needed to be balanced with protection of liberties and minimising the inconvenience to passengers in our airports.
28 days
Vigilance
He said a new forum of headteachers would be convened to find ways to protect pupils from extremist propaganda.
Lord West told Today that he started his brief "on the premise that what we mustn't do is the terrorists' job for them".
"There is no greater priority than the safety and security of our people and building the strongest possible relationships across all faiths and communities," Mr Brown told MPs.
"Yes, there is a great risk, we've got to be vigilant - but we need to make sure we conduct our normal way of life in terms of business, work, pleasure, all these things," he said.
"And I believe it possible to build a stronger consensus that will both root out terrorist extremism and build more vibrant and cohesive communities."
"There is no doubt there are very real threats, that the design in the past of certain buildings and certain places does not make it easy to counter a terrorist attack on them.
Mr Brown's statement came amid a growing furore over Lord West's comments.
"If a bomb goes off, we have actually built into them the shrapnel that will kill and injure people and we can actually do things to make this better.
He initially told the BBC at 0820 on Wednesday he was not convinced about extending the limit beyond 28 days - in apparent contradiction of recent ministerial comments.
"We very clearly, from now on, must always make sure we design in counter terrorism measures as we do counter crime measures."
But then an hour later, after talks in No 10 with the prime minister, he told the BBC that he was convinced of the need to extend the limit.
'Cancer of terrorism'
The peer later insisted he had not changed his position. "Maybe being a simple sailor and not a politician I didn't choose my words very well," he said.
The peer said he and his team had looked at how to protect existing buildings, by redesigning road layouts, putting in barriers and using specific types of glass.
But Conservative leader David Cameron said it looked as if Lord West had been "leant on" by Mr Brown.
He said Britain "can spend the national wealth on protecting ourselves and trying to chase these criminals down".
Lib Dem home affairs spokesman Nick Clegg called the chain of events "Keystone Cops politics".
But none of the measures would be effective without "exorcising this cancer of terrorism", by "stopping the radicalisation of our youth" - a measure the prime minister is expected to update MPs on.
Details of the unified border force, first announced by Mr Brown in July, are also due to be outlined on Wednesday.
A study led by the Cabinet secretary, Sir Gus O'Donnell, considered whether it should include police officers as well as those from the Border and Immigration Agency, Customs and the overseas UK Visa operations.
