Mexico's Congress has approved a controversial labor overhaul bill introduced in September by outgoing conservative President Felipe Calderon.

The Senate passed the bill by a vote of 99-28 on Tuesday and it will now go to the president for his signature. The lower house had given its approval for the bill in late September.

The measure would allow employers to pay hourly wages; formally regulate outsourcing, a practice that has become widespread in Mexico in recent years; and institute probation periods for new employees.

Provisions in the bill that would have made several powerful unions linked to the Institutional Revolutionary Party, or PRI, more democratic had been stripped out before the final Senate vote.

The union transparency provisions included a requirement that labor leaders be elected in free and secret balloting, as opposed to by a show of hands.

The center-left PRI has traditionally relied on union support, especially during its 71 years of uninterrupted rule between 1929 and 2000.

The bill was passed two weeks before the PRI's Enrique Peña Nieto takes office as Mexico's next president.

In September, Mexican academic Angel Lopez Montiel criticized the departure from the current system of a set monthly wage.

"Hourly pay might look like the same system that is used in the United States," Monterrey Tech researcher Angel Lopez Montiel told Efe then.

The difference, however, is that hourly pay in the United States is much higher than in Mexico, he added.

Prior to the passage of the bill in the lower house on Sept. 29, leftist lawmakers stormed the speaker's platform and forced supporters of the bill to lead the session from a spectators' gallery.

The controversial overhaul was the first order of business for the Congress that emerged from Mexico's July 1 general elections.

The current labor law was enacted on April 1, 1970. The bill passed Tuesday marks the first successful bid to change that legislation after hundreds of other attempts had failed. EFE