Twenty-six members of Congress, including 21 senators, have written a letter to South African Ambassador Herbert A. Beukes protesting his government's plan to incorporate several pieces of land into one of the 10 homelands set aside for black South Africans and to grant another of them "independence" next year.

The letter, dated Dec. 18, said that the plan to incorporate the Moutse area into the homeland of KwaNdebele, north of Pretoria, and to grant the homeland independence directly contradicted what South African President Pieter W. Botha said in a speech Sept. 30.

In that speech, Botha said the white-ruled South African government was committed to "one citizenship and a universal franchise within a united South Africa" and to the restoration of South African citizenship to those blacks living in the homelands who had earlier lost it.

The letter, initiated by Sens. John Glenn (D-Ohio) and William V. Roth Jr. (R-Del.), said the signatories understood the government had not taken into account the wishes of the people of Moutse, or another area, Ekangala, both of which are scheduled to be included in the KwaNdebele homeland.

"We have long considered the 'homeland' policy one of the most inhumane manifestations of apartheid," the letter said. "To transfer the people of Moutse and Ekangala, against their will, to KwaNdebele, and thus deprive them of their South African citizenship is tantamount to a forced removal and a most telling example of the injustice of the 'homeland' policy."

Among the other signers are Sen. Nancy Landon Kassebaum (R-Kan.), head of the Senate Africa subcommittee; Sen. Claiborne Pell (D-R.I.), the ranking minority member on the Senate Foreign Relations Committee; Sen. Edward M. Kennedy (D-Mass.); Rep. Dante B. Fascell (D-Fla.), chairman of the House Foreign Affairs Committee, and Rep. Howard E. Wolpe (D-Mich.), chairman of the House Africa subcommittee.

The legislators urged the South African government to reconsider its plan to incorporate the two areas into KwaNdebele and instead to hold a referendum to determine whether the residents agree to it.

Beukes was not available for comment on the letter.

Several western countries, including the United States, have expressed to the South African government their opposition to the plan to grant KwaNdebele its independence, according to a U.S. official.