A Mexican congressional working group has gutted a planned reform of several measures intended to make trade unions more transparent ahead of a vote on the bill later this week.

Put forward by outgoing President Felipe Calderon at the start of the month, the draft labor law aims to make it easier for companies to hire and fire employees and combat abuses by the country's trade unions.

Viewed as a litmus test of cooperation between Calderon's conservatives and the centrist Institutional Revolutionary Party (PRI) of incoming president Enrique Pena Nieto, the bill must be voted by the lower house of Congress this month.

Calderon's original proposal had sought to impose external audits on the unions, force them to divulge details of their balance sheets to members, and give workers a legal right to seek suspension of the fees they must pay to unions.

The revised bill, seen by Reuters on Tuesday, replaces these terms with a general pledge to impose "rules on accountability" that will "consider" the right of workers to receive information on how funds are spent and procedures to resolve disputes.

The new document, which could still be revised again, also strips out a clause that insisted the election of union leadership would have to be free, direct and secret.

The changes reflect the PRI's desire to defend the power of unions, which have been a keystone of support for the party that long dominated Mexico. But critics say too many unions are corrupt, squander funds and must be opened up to greater scrutiny.

Lawmakers from the PRI and the PAN have said a labor bill is likely to be approved. If it passes the lower house, which could come as soon as Thursday, it must still go through the Senate.

The PRI ruled Mexico for 71 consecutive years between 1929 and 2000 before its defeat by the National Action Party (PAN) under Vicente Fox, Calderon's predecessor as president.