In a move that appeared to open competition for air rights to the Metropolitan Transportation Authority’s West Side rail yards, the MTA chairman, Peter Kalikow, sent letters to the New York Jets and Madison Square Garden – currently the only organizations that have placed bids on the site – asking for the submission of “best and final” bids by March 21.

That is the deadline for all bids, a spokesman for the MTA, homas Kelly, said. All proposals will be reviewed and voted on at a meeting of the full board of the MTA on March 31.

Acquisition of the air rights, or development rights, by the Jets are assumed in the Bloomberg administration’s plan to create the New York Sports and Convention Center above the Long Island Rail Road yards by the Hudson River. That facility, featuring a domed football stadium, would also be the principal venue if New York is selected this July as host city for the 2012 Olympics.

Next Tuesday, information will be posted on the MTA Web site detailing the specific requirements for submitting a bid, Mr. Kelly said last night. One of them, he said, is presenting a letter of credit proving that bidders have the financial resources to follow through with their development proposals.

Until February 4, the Jets, aggressively courted by the Bloomberg and Pataki administrations, had been the only entity to make a bid for the rail yards’ development rights. They offered $100 million for the property. The cost of the $300 million-plus deck that would need to be installed before construction could begin, as well as the stadium’s retractable dome, would be covered by equal contributions of $300 million by the city and the state.

That Friday, however, Madison Square Garden, owned by Cablevision, announced that it was offering the MTA $600 million for the development rights, a sum that would be reduced by whatever it cost to build the deck. Proponents of the Jets’ stadium plan dismissed the offer as a PR gimmick meant to maintain the Garden’s “monopoly” on large sports facilities in Manhattan, and estimated the current value of the offer at closer to $60 million. The addition of another bid, the MTA’s Mr. Kelly said, necessitated imposing a deadline. “We want to make sure the process doesn’t drag out forever, so the MTA doesn’t lose out on revenue,” he said.

Mr. Kelly said the MTA board is not bound to accept the highest bid, adding that the authority would take into consideration the proposed construction on the site in making its decision. Mr. Kelly also said the announcement should not be seen as a new opening of the bidding process, which, he maintained, is now “as open as it has been all along.”

A Westchester Democrat in the state Assembly, Richard Brodsky, who has held hearings into the city’s stadium deal with the Jets and advocated an open bidding process, called Mr. Kalikow’s action “courageous” and said that, “to his credit,” the MTA chairman “has chosen to show his loyalty to the people who ride the subways, buses, and trains.” Mr. Brodsky said they should be the foremost concern for the MTA as it looks to sell the site.

A spokesman for the Jets, Marissa Shorenstein, responded to the MTA’s announcement with a statement: “In the end, a Cablevision gimmick on the eve of the Olympic evaluation committee visit will not replace four years of planning and an agreement to build the world’s greatest sports and convention center in New York City.”

Similarly confident that the Jets’ stadium plan would go through was the deputy mayor for economic development, Daniel Doctoroff, who has been the point man for the Bloomberg administration’s sweeping plan to renew the far West Side and its effort to win the host-city designation for 2012. “We are confident that the MTA’s process will determine that the Jets’ offer is far superior to MSG’s PR stunt,” Mr. Doctoroff said in a statement. “… Once this process is complete, we look forward to moving forward with the project that will generate an enormous profit for the city and state, and help pay for vital city services like education and public safety.”

The leader in polls on this year’s mayoral race, Fernando Ferrer, issued a statement that said: “On February 1st, I called on the M.T.A. to open the bidding process to guarantee New York?s straphangers get the best price for this land. I’m glad that they agreed. It’s good to see that the M.T.A. has decided to stand up to Mayor Bloomberg and open up the bidding process. This is a victory for good government and New York’s straphangers and a defeat for sweetheart deals made behind closed doors. Let the market decide the best price for this valuable public asset.”

A spokesman for Madison Square Garden declined to comment on the MTA letter.

Earlier in the day, before the MTA’s announcement, Mayor Bloomberg had been quoted as saying there would be “a shovel in the ground” to build an Olympic stadium on the West Side by the time the International Olympic Committee announced a host city for the 2012 games July 6.