Alaska Air Group - parent company of Alaska Airlines - is to merge with Virgin America.

Under the agreement, the group will acquire Virgin America for US$57 (€50) per share in cash in a deal worth approximately US$4bn (€3.5bn).

The combination will make Alaska Airlines the fifth largest US carrier.

"The combined airline will also offer more frequent connections to international airline partners departing Seattle, San Francisco and Los Angeles", Alaska Air Group says in a statement.

Alaska Airlines adds that it "expects" Virgin America's pilots, inflight staff and maintenance technicians will be protected.

Virgin America was launched in 2007 by founder Richard Branson.

In a statement Mr Branson said: "I would be lying if I didn't admit sadness that our wonderful airline is merging with another".

"Because I'm not American, the US Department of Transportation stipulated I take some of my shares in Virgin America as non-voting shares, reducing my influence over any takeover. So there was sadly nothing I could do to stop it".

But he added: "Despite the turbulence and head winds, the journey remains thrilling and joyful, and I look forward to more future flights with Virgin America".

The merger has been approved unanimously by the boards of directors of both companies, but is subject to regulatory approval.

The companies say they expect to complete the deal no later than January 1st, 2017.

Virgin America has been voted 'Best Domestic Airline' in both Travel + Leisure's Annual World's Best Awards and Conde Nast Traveler's Readers' Choice Awards for the past eight consecutive years.

While Alaska Airlines has been ranked 'Highest in Customer Satisfaction Among Traditional Carriers' by J.D. Power for eight years running.