The government contends the acquisition of the No. 4 wireless carrier in the country by No. 2 AT&T would reduce competition and raise prices.
The deal has faced opposition from consumer groups and No. 3 carrier Sprint.
The official spoke on condition of anonymity in advance of the public announcement.
A failure of the deal would put T-Mobile in a difficult position. It's struggling to compete with the larger carriers, and owner Deutsche Telekom AG has said it's not willing to invest more in the venture.
AT&T has promised T-Mobile $3 billion in cash if the deal doesn't go through, plus spectrum rights and agreements that could be worth billions more.