Google is suing the US government, saying it was unfairly excluded from a $58m deal to revamp e-mail systems at the Department of the Interior.
In a lawsuit filed with the US Court of Federal Claims, Google says the terms of the five-year contract rule out its products and favour rival Microsoft.
The search giant wants to offer its Google Apps software for the contract.
But Google says it was told that only Microsoft's business software could be used - a move it called "arbitrary".
The Department of the Interior, which employs about 88,000 people, declined to comment on Google's legal action.
Google says in its lawsuit that it was told there would be "full and open competition" for the contract, but that the bid specified that only the Microsoft Business Productivity Online Suite-Federal could be used.
This is a special version of Microsoft's suite of business tools that was designed for the US federal government.
Google has its own special version of its software designed to address government security concerns, Google Apps for Government.
However, the firm says it was told its product did not comply with the department's security requirements.
Google says the decision was "arbitrary and capricious", as well as being "unduly restrictive of competition" and against US law.
It calls for an open contracting process, which it says "could save US taxpayers tens of millions of dollars and result in better services".