Police in training for 'Mumbai-style' gun attack in UK
UK security chiefs have ordered an acceleration in police training to prepare for any future "Mumbai-style" gun attack in a public place.
A series of counter-terrorism exercises are being held with police and the SAS.
The BBC's security correspondent Frank Gardner said there was no indication such an attack was imminent in the UK.
The Home Office said: "We keep security arrangements under constant review to take account of the threat, lessons we have learned and new challenges."
Last month intelligence sources said they had uncovered the early stages of an al-Qaeda plot to carry out co-ordinated attacks in the UK, France and Germany.
Suspects were planning to copy the 2008 attacks in the Indian city of Mumbai, where 10 gunmen went on a three-day rampage, killing 166 people and injuring more than 300, the sources said.
In response police armed response units are being given more powerful weapons.
Our correspondent said the UK authorities had been planning for such an attack ever since Mumbai happened.
"David Cameron has taken a personal interest in the problem ever since his first threat assessment given to him when he took office in May.
"Now police armed response units are getting their firepower and their stocks of ammunition increased to deal with multiple terrorists armed with automatic weapons," he said.
A Whitehall official told the BBC the Metropolitan Police had not been asked to to do this before.
He said the job of the police would be to contain such a situation while the job of the SAS (Special Air Service), if called upon, would be to resolve it.
Former Security Minister Lord West told the BBC the police must be properly trained to deal with such terror attacks.
"These people like the Mumbai terrorists, are a bit like soldiers, they do fire and support, move forward, all they want to do is kill as many people as possible, with slightly heavier weapons than our police have," he said.
"And therefore you have to give heavy weapons to the police and train them how to do it.
"There is no way, except at immense cost, you could have SAS-level trained troops in every part of the country to be able to respond in the timescale you'd need. The police are there and have to do that first response."
Meanwhile, plans are being drawn up for an SAS unit to be stationed in London throughout the 2012 Olympics.
A Home Office spokesperson said: "We do not comment on intelligence or operational matters, or specific threats. The police regularly train and exercise for a variety of scenarios with a variety of partners.
"The police have established procedures in place to deal with a terrorist incident which are constantly updated. It is right that we learn the lessons from previous incidents and that these inform and strengthen such procedures.
"We know we face a real and serious threat from terrorism. The overall threat level, set by JTAC (Joint Terrorism Analysis Centre), remains at severe which means that an attack is highly likely. This has not changed."