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 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 that 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," Pritchard said.

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 United Kingdom thanks to public calls.

An analysis of figures released to the Associated Press under Britain's Freedom of Information law detailing phone calls to a police antiterrorism hotline shows a baffling correlation between government warnings and the public response in the United Kingdom.

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 United States, 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 convince people that they need to show extra caution.