A prominent business group on Tuesday penned all members of the Senate to voice its unhappiness with the Senate health bill.

The American Benefits Council (ABC), which represents many large employers, wrote that it is still "unable" to support healthcare reform legislation despite a key compromise granted to centrists by Senate Majority Leader Harry Reid (D-Nev.) on Monday.

“We are unable to support the Senate bill in its current form and are increasingly disturbed that significant progress has yet to be made to address the problems it poses for employer-sponsored health care," he group's president James A. Klein wrote. "Without significant improvements, we will have no alternative but to oppose the legislation as it is considered further by the Senate."

The letter comes a day after Reid agreed to drop the proposed Medicare buy-in from the bill. The measure would have allowed people between the ages of 55 and 64 to purchase Medicare coverage in a bid to expand healthcare coverage.

Reid dropped the provision in part to win the support of centrists, such as Sen. Joe Lieberman (I-Conn.) who said they oppose the buy-in. It was reported last week that Reid dropped the public health insurance option from the bill as well. The buy-in was partly meant to be a concession to liberals who support the public option.

But ABC believes the bill will still up costs for employers that provide healthcare coverage to their workers. The letter presses senators to make the excise tax on high-cost plans "fairer."

The letter notes that the group opposes the Medicare buy-in and the public option despite the fact that they appear to have been stripped from the bills.

ABC, in a release, describes itself as "a constructive voice throughout the health care debate" and says it "remains one of the last business organizations still working to make the current health reform process succeed."