A major Northern Virginia bingo operator yesterday pleaded guilty to two counts of illegal gambling in a surprise development in Alexandria Circuit Court.

Special Prosecutor Edward J. White, who had several witnesses present and ready to testify, later dropped three remaining gambling counts against the operator, Alva Ford Thompson.

Thompson, whose indictment in August followed an undercover investigation by an Alexandria policewoman, faces a maximum of 20 years in prison and $40,000 in fines.

White yesterday told Circuit Court Judge Franklin P. Backus that, as a result of discussions with Thompson's lawyers, he would recommend that Thompson receive "at least" two years in prison and a $30,000 fine.

Backus postponed sentencing until Feb. 9 pending a probation report. Thompson remains free on his own recognizance.

White told Backus that had the case gone to trial he would have shown that Thompson at all times kept three sets of books for the various bingo games he conducted on behalf of charities -- games which grossed $282,000 from December 1977, to May, 1978. White said that of this total, nearly $65,000 in cash could not be accounted for "by any conceivable auditing procedure."

Thompson pleaded guilty to conducting illegal gambling operations in the guise of bingo from Jan. 26, to May 7, 1978, on behalf of St. Paul's Pentecostal Church, in Alexandria. Thompson also pleaded guilty to conducting illegal gambling operations between Feb. 5 and Feb. 9, 1978 "for himself, without any semblance of charitable operation or purpose." The location for both illegal gambling operations was the Arlandria bingo auditorium, 3819 Mt. Vernon Ave., Alexandria, White said.

Virginia law requires that bingo games be conducted by unpaid volunteer members of the sponsoring organizations, with all proceeds except overhead going to the sponsoring charitable groups.

White told Backus that the bingo games conducted in the name of St. Paul's Pentecostal Church grossed nearly $77,000, although the church received only $2,818.

White said that from Feb. 5 to Feb. 9, 1978, Thompson conducted bingo games in the names of charitable organizations which had lost their bingo permits, or without the names of any charitable organizations at all. One of Thompson's three sets of records showed that no bingo games were conducted by him on Feb. 6, although another set showed that Thompson had netted $600, White said.

The counts which were dropped involved bingo games sponsored in the names of the Alexandria Volunteer Fire Department, the Alexandria YMCA and the Liberty Rebekah Lodge, according to last summer's indictment.

Thompson is scheduled to stand trial on Jan. 16 in Fairfax County Circuit Court on similar charges of illegal gambling.