In an important strategic move, Texas International Airlines made a formal merger proposal to Continental Airlines, the Los Angeles-based airline announced today.

In a letter from Frank Lorenzo, chairman of TI's executive committee, delivered to Continental shortly after its annual meeting in Denver, TI offered to pay $13 cash for all remaining shares of Continental stock it doesn't already possess.

Alternatively, TI offered what was described as a tax-free reorganization that would provide each remaining shareholder with equity securities in a combined company that would be valued at $14 a share, a value that would be agreed to by investment bankers of both companies.

The Houston-based airline also said it would be prepared to accept standard labor protection provisions that would protect Continental employes financially should the merger lead to any dismissals or displacements or require any employes to move.

The offer, open until May 21, is conditional on financing that TI said it expects to get by May 19.

Under the offer, which already has invested $93 million in Continental stock to gain its 48 1/2 percent holding, Texas International would have to spend another $115 million to get control of the company if its cash offer is accepted.

A.L. Feldman, Continental's president and chief executive officer, said TI's offer was referred to a special committee of its board's outside directors that was created to evaluate Continental's plan to create an employe stock-ownership plan.

In a related development, Texas International today suspended four of its pilots, pending an investigation for alleged violations of their contracts. The four were charged with disrupting other employes in their work.