The Port Authority bus terminal, long considered one of Midtown’s most prominent eyesores, is one step closer to becoming a base for a soaring new skyscraper after the board of the Port Authority of New York and New Jersey released three possible design plans for the building yesterday.

A London-based firm, Rogers Stirk Harbour & Partners, submitted the most daring of the proposals, which were formally presented to the agency’s board yesterday. Its design features three stacked boxes held together with diagonal trusses rising 42 stories, and includes more than 40,000 square feet of retail space. The other two plans, by Kohn Pedersen Fox Architects and Pelli Clarke Pelli Architects, envision 48 and 47 stories, respectively.

Port Authority officials said the final design would be chosen in the next two months and that construction would begin in late 2009 or early 2010.

A joint venture of Vornado Realty Trust and the Lawrence Ruben Company will build a 40-story, 1.3 million-square-foot office tower above the north wing of the existing bus terminal. The developers also are proposing making significant renovations to the terminal, including 18 new departure gates and an overhaul of about 60,000 square feet of its retail space.

The chairman of the Port Authority’s board of commissioners, Anthony Coscia, said the project would improve commutes for bus riders, as the agency would reap about $500 million over the term of the lease.

The board also approved an agreement between the Port Authority and the New York Police Department to create a specialized World Trade Center police unit that will work in conjunction with Port Authority police to provide security for the site.

“Today’s agreement provides a clear delineation of responsibility so that the city and the Port Authority can work together to ensure seamless security that will keep the World Trade Center site safe,” Mayor Bloomberg said in a statement.