WASHINGTON — A bill intended to improve the finances and operations of the Postal Service won Senate approval Thursday despite last-minute opposition by management of the agency it is intended to help.

The Senate bill was passed by voice vote. The House passed similar legislation by a vote of 410-20 in July. The two versions will now go to conference committee to work out differences.

It's crucial to change the way post offices operate, the sponsors of the Senate bill said in a joint statement. "Under its current business model, the Postal Service's financial future is not viable," said Sens. Susan Collins, R-Maine and Thomas Carper, D-Del.

More than 9 million American jobs, $900 billion in commerce and 9 percent of the gross domestic product depend upon mail and package delivery.

While postal management has worked with Congress for several years to get reform legislation, late last month the postal governing board announced opposition to the measure.

"We believe there are critical elements missing from this bill, as well as numerous burdensome provisions that would make it extremely difficult for the Postal Service to function in a modern, competitive environment," the Postal Service's governing board said in a letter to Collins, who chairs the Senate Homeland Security and Governmental Affairs Committee.

Top postal officials could not be reached Thursday for comment on passage of the measure.

The bill seeks to improve postal finances, but also gives more power to the independent Postal Rate Commission, changing it to a regulatory agency with authority to rule on postal rates and other activities.

Major provisions of the measure, which the post office has sought, include relief from a requirement to place more than $3 billion each year in an escrow account and transfer certain retirement costs back to the Treasury.