The Bush administration said on Friday it would permit the largest ever U.S. airline marketing alliance to take effect, but imposed conditions.

After reviewing the proposal for nearly six months, the Transportation Department cleared the codeshare proposal by Northwest (NasdaqNM:NWAC - News), Delta (NYSE AL - News) and Continental (NYSE:CAL - News) airlines, a government source said.

But the source said the carriers must adhere to a series of conditions, which the official would not disclose.

Regulators cleared a similar deal between bankrupt carriers United Airlines (NYSE:UAL - News) and US Airways (OTC BB:UAWGQ.OB - News) last year and imposed conditions on overlapping routes.