President George Bush's practice of writing exceptions to legislation as he signs it into law represents a violation of the constitution and a danger to democracy, America's leading lawyers alleged yesterday.

The American Bar Association, an independent lawyers' organisation, issued a report on President Bush's prolific use of "signing statements" and found he was using them to create unconstitutional loopholes to laws passed by Congress.

The ABA found that the president used signing statements to make more than 800 challenges to congressional legislation, 200 more than all previous US presidents put together. Signing statements have been issued since the nation's founding but they have traditionally served a ceremonial function, extolling the virtues of the legislation just signed.

Mr Bush has used the statements to distance himself from the laws he signs rather than veto them outright. A veto can be overruled by Congress but the legislature has so far been powerless to make the White House enforce provisions the president does not like.

For example, in signing a bill last year banning the use of torture by American personnel, the president wrote that the executive branch would "construe" the legislation "in a manner consistent" with the president's powers and "the constitutional limitations on the judicial power". In other words the president would not enforce the law if it conflicted with his authority as commander-in-chief to pursue his "war on terror" as he saw fit.

"This report raises serious concerns crucial to the survival of our democracy," the ABA's president, Michael Greco, said. "If left unchecked, the president's practice does grave harm to the separation of powers doctrine, and the system of checks and balances that have sustained our democracy for more than two centuries."

Asked about the report yesterday, the White House spokesman, Tony Snow, said: "The president does not have the luxury of practising civil disobedience. The laws that have been enacted must be executed by the government. A great many of those signing statements may have little statements about questions about constitutionality. It never says, 'We're not going to enact the law.'"

The justice department has denied that Mr Bush's use of signing statements has gone significantly beyond previous presidents. However, the Bush administration subscribes to a constitutional theory known as the "unitary executive". It stipulates that under the constitution, the president has supreme control over all executive branches of government and that Congress has very limited power to divest him of that authority.

The ABA report found that the administration's interpretation of the "unitary executive" theory violated the constitution's strictures on the separation of powers, that were "essential to the preservation of liberty".

"The constitution thus limits the president's role in the lawmaking process to the recommendation of laws he thinks wise and the vetoing of laws he thinks unwise," it said.

The president's attempts to gain the power to issue "line item vetoes" against individual provisions within congressional bills, have been rebuffed by Congress. The ABA called on Congress to pass a bill allowing signing statements to be challenged in the courts.

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