United Airlines, American Airlines and Delta Airlines will call on the US authorities to block the deal on the grounds that it would be a step towards the creation of a 'global airline monopoly'.
The call, to be made at a joint press conference in Washington by the three airlines and Federal Express, demonstrates the seriousness with which they view the threat from BA.
The US carriers claim that if the deal is approved it will lead to thousands of job losses, weaken the competitive position of US airlines and undermine moves to greater freedom of the skies.
Opposition to the deal has been gathering pace since BA announced in July that it had reached agreement to take a 49 per cent equity stake and 21 per cent of the voting rights in USAir, the country's fifth biggest carrier.
With the agreement now before Andrew Card, Transportation Secretary, for formal consideration, the lobbying has reached a crescendo.
Until now United and American have taken the view that the tie-up could be allowed provided they won reciprocal rights to operate transatlantic services to more regional UK airports.
Stephen Wolf, United's chairman, flew into Birmingham yesterday saying that if US-UK air services were liberalised it would begin services to the airport from Chicago and Washington using twin-engined Boeing 767s.
Delta, however, has always maintained that BA's proposed investment should be blocked no matter what on the grounds that it infringes US aviation law.
BA exacerbated the row this week by announcing that it would start services from Birmingham to New York next spring. Under the existing air services agreement between the two countries US airlines have no rights available to increase flights to regional airports other than Glasgow.