By Barbara Plett BBC correspondent in Jerusalem

Palestinian officials say Yasser Arafat has finally signed a de facto constitution that defines his powers.

William Burns has just arrived in the Middle East to renew diplomatic efforts

Mr Arafat has ignored the Palestinian Basic Law since it was adopted by legislators five years ago, but he has come under increasing pressure at home and abroad to institute political reforms.

The Palestinian Basic Law is essentially a constitution that details the division of powers and the rights and freedoms of Palestinian citizens.

Legislators have been asking Mr Arafat to sign it into effect since its adoption by legislators.

Public dismay

Now he has finally done so, without announcement, according to Palestinian negotiator Saeb Erekat.

The move comes after unprecedented internal pressure for reform.

Public dismay over the perceived corruption and incompetence of Mr Arafat's Palestinian Authority boiled over after Israel's recent six-week military campaign in the West Bank.

Palestinians accused the authority of failing to defend or help them and demanded changes.

Significantly, so has the United States, and a top envoy, William Burns, has just arrived in the region to renew a diplomatic initiative.

Measures promised

In a recent speech, Mr Arafat promised measures to guarantee the separation of powers, judicial independence, elections and reform of the security forces.

Signing the Basic Law is a prerequisite to these things.

However, the Palestinian leader has consistently resisted calls to loosen his autocratic grip on power in the past.

It would be a departure from standard practice if he effectively implemented the law.