Hours before the Postal Service's expected first-ever default on an obligation, senators blasted the Republican-led House of Representatives for not advancing legislation to overhaul the cash-strapped service before a month-long recess.

The mail agency, which relies on the sale of stamps and other products rather than taxpayer dollars, confirmed this week that it could not make a $5.5 billion payment for future retiree health benefits that is due by midnight on Wednesday.

Lawmakers have said for more than a year that they would overhaul the struggling agency and help it avoid a default.

The Senate passed a bipartisan bill in April. But the House plans to leave at the end of this week until after the September 3 Labor Day holiday without voting on postal legislation.

"Changes are needed now, but we cannot move forward without action by the House of Representatives," Republican Senator Susan Collins said in a statement on Wednesday. She and the other three authors of the Senate bill urged the House to act.