The proposed tie-up between British Airways and American Airlines has hit a stumbling block after two senior US transport chief have been forced to rule themselves out of the decision-making process.

John Porcari, the deputy US transportation secretary, and Ray LaHood, the US transport secretary, have been forced to rule themselves out of the decision whether to grant BA anti-trust immunity.

According to The Telegraph, Porcari ran the risk of leaving himself open to accusations of bias after it emerged he had signed a letter last November, before taking office, in support of the proposed alliance.

The letter to former US Transportation Secretary Mary Peters was written while Mr Porcari was in his previous role as Maryland Transportation Secretary.

It urged support for the BA/AA tie-up on the basis that it would boost traffic through Baltimore Washington International Airport. The letter claimed approval was “critical for the future growth of transatlantic travel” through the airport.

Porcari joins LaHood who exempted himself after saying that “these alliances are life-savers for airlines”.

The department has until October 31 to decide whether to approve the tie-up.