A liberal-leaning think tank urged the Trump administration on Thursday to block T-Mobile’s $26.5 billion acquisition of Sprint, arguing that the merger would concentrate too much market share among just three telecommunications firms.

“A combined T-Mobile and Sprint means higher prices and worse service in wireless phone service,” Barry Lynn, executive director at the Open Markets Institute, wrote in a brief to the Federal Communications Commission. “This is true for the public at large, and especially true for rural and suburban communities.”