Kirsty Wigglesworth, File, Associated Press
FILE - In this March 31, 2009 file photo, employees are silhouetted by screens showing prominent areas of London under CCTV surveillance at a police control room in London. Britain has announced in advance it will raise its terrorism threat level during the London Olympics next summer, but that could be the last time the five-point scale is used amid mounting evidence such systems are often misunderstood and do little to generate crucial tips about terror plots.

LONDON — Britain has announced in advance it will raise its terrorism threat level during the London Olympics next summer, but that could be the last time the five-point scale is used amid mounting evidence such systems are often misunderstood and do little to generate crucial tips about terror plots.

Data obtained by The Associated Press under a Freedom of Information request show terror tips from the public have consistently fallen when alerts are raised and risen when the scale is lowered, confounding expectations that boosting threat levels promotes greater vigilance.

Just as in the United States over recent months, British officials are agonizing over how to keep the public alert, but neither gripped by fear nor dismissive about supposed government scare tactics.

Mark Pritchard, a Conservative Party lawmaker and member of Britain's National Security Strategy Committee, said many legislators and security officials now agree the alert scale is flawed, reflecting new research over how people respond to the risk of terrorism.

"The system is too complex and misunderstood by many members of the public, who struggle to decipher between the various alert stages," said Pritchard.

Pritchard and others say that is a concern because of the importance of public tips in halting terrorist plots.

A 2010 study of 68 thwarted U.S. terror plots by North Carolina's Institute for Homeland Security Solutions showed the public provided initial clues in 29 percent of cases where attacks were halted between 1999 and 2009. That was more than the number uncovered by intelligence agencies, and only slightly less than the 30 percent detected by federal agents.

British police declined to provide details of the number of plots thwarted in the U.K. thanks to public calls.

An analysis of figures released to the AP under Britain's Freedom of Information law detailing phone calls to a police anti-terrorism hotline shows a baffling correlation between government warnings and the public response in the U.K.

Since 2006, public input has dropped off on all but one occasion when the threat level has been raised, even though tips are likely to be most important during times when the threat is unusually high.

Every time the threat level has been lowered over that period, the volume of calls has risen.

Even when Britain's threat level was raised in June 2007 to its highest setting of critical — meaning an attack is deemed imminent — the number of calls to the hotline fell. On both recent occasions when officials have lowered the threat level, first in September 2010, and again in July, the number of calls has increased.

Risk perception expert David Ropeik, who advised the Department of Homeland Security on changing the threat level system in the U.S., said the public's response to the terrorist threat was rarely logical.

By publicizing a decision to lower the threat level, governments may just remind people about the potential dangers — generating more, not less, alarm. Equally, a decision to raise the alert status which includes little specific detail often fails to persuade people that they need to show extra caution.

How people perceive the threat from terrorism is "a combination of the few facts we may have, and how those facts feel," rather than an exercise in rational thinking, Ropeik said.

Concern over Britain's five-point system has been heightened after ministers announced in July that their planning for the Olympics assumed Britain's threat level will be raised to severe — the second highest setting, indicating that an attack is considered highly likely.

If a change can be announced 12 months in advance, and based on assumptions rather than specific intelligence, some wonder if there is any value at all in the alert system.

Officials insist the announcement for the London games didn't indicate that they had gathered so-far-undisclosed intelligence about planned attacks, but reflects an expectation the event will be a major target. Security at the Olympics has often erred on the side of caution, particularly since the slaying of 11 Israeli athletes and coaches at the 1972 Summer Games in Munich.

For most countries, alert scales were originally intended as a guide only for security specialists in specific industries, not for the public. The levels usually trigger official responses such as additional screening checks at airports, or restricting vehicle access to sensitive buildings, but don't require any action from ordinary citizens. Britain first issued a public threat scale in 2006.

Home Office minister James Brokenshire has acknowledged Britain's five-point alert scale is under review, but has not specified when it might be changed. Other government officials say an overhaul will begin soon after the Olympics.

To add to the current confusion, Britain no longer has a single overall threat level. It issues separate assessments of the threat from al-Qaida-related terrorism, and for risks linked to dissident Irish Republican Army members. In recent months, it has even published separate alert levels for the threat to mainland Britain, and to Northern Ireland.

Britain's Joint Terrorism Analysis Centre, based at the MI5 spy agency, also issues at least a dozen other secret threat levels — which are not made public — to security experts in aviation, finance, infrastructure and other sectors.

The most frequently cited public level, the risk of international terrorism to the U.K. mainland, is currently ranked as substantial — the third highest setting, explained as meaning that an "attack is a strong possibility."

Authorities in the U.S. previously used a similar, color-coded five point scale, but in April overhauled the system, mindful that it offered little public reassurance. New alerts distinguish between imminent or elevated threats to specific locations or industries, offer guidance on how the public should respond and indicate when the increased threat is likely to pass.

"The government has a difficult dance to do balancing public safety and the terrorist risk," said Ropeik, author of "How Risky Is It, Really?"

"On one hand they do want us to be alert and vigilant. On the other hand, they do not want to be seen to be crying wolf, or cynically trying to manipulate our worries to advance their political agendas," he said.

Research conducted by Britain's interior ministry has found many British Muslims believe the government exaggerates the risk of terrorist attacks to justify "aggressive domestic policies against U.K Muslims," particularly searches by police, airport and customs staff.

Polling carried out in 2009 for Britain's government by IPSOS-Mori found almost half of those questioned believed ministers were not honest in communications about terrorism.

Gurdeep Kaur, a 36-year-old hospital worker, claimed many Britons simply aren't concerned by fluctuations in the country's alert scale.

"I feel safe regardless, I don't know how they evaluate it, but it doesn't affect day to day life," said Kaur.

Underlining the confusion which experts say shows the system should be overhauled, Kaur said she believed Britain's alert level had been raised in recent weeks — when in fact it was downgraded in July.

Associated Press writer Sylvia Hui in London contributed to this report