Mayor Jane M. Byrne, with no warning and only a hint of her characteristic spunk, tonight abandoned her week-old and weakly supported write-in candidacy in the April 12 Chicago mayoral election.

The immediate reason she cited for her withdrawal was technical: the city elections board today rejected her attempt to change Chicago's awkward write-in process, and Byrne said tonight that a court fight would be long, cumbersome and confusing.

"Due to these factors, it has become clear that a successful write-in campaign cannot be properly executed during the short three-week period before election. For these reasons, I am withdrawing as a write-in candidate for mayor," Byrne said in a short written statement delivered to a local news service by a press aide.

It had been apparent for days that Byrne was unable to rally support from key sectors of the shattered Democratic organization, unable to assemble a credible campaign staff and barely able to muster any large contributions.

In her statement, Byrne, who finished second in the bitter three-way Democratic primary Feb. 22, did not endorse either the Democratic nominee, Rep. Harold Washington (D-Ill.), or the Republican nominee, Bernard E. Epton, who is backed by many Democrats lukewarm to Washington's bid to become the city's first black mayor.

She said only, "I thank all those who encouraged the write-in and who have so strongly supported me. However, there can be no write-in and I urge no token write-in to take place. I encourage everyone to please vote on April 12."

Byrne's reentry into the race had been viewed generally as a boon to Washington, the near-unanimous choice of Chicago blacks, who make up nearly one-third of the city's registered voters. He would have benefited from a split in the white vote.

Epton's camp immediately welcomed Byrne's withdrawal, predicting that he would get many of Byrne's supporters.

The Byrne announcement came on the eve of a critical meeting of the city's 50 powerful ward committeemen called by Cook County Democratic Chairman Edward Vrdolyak, formerly a key Byrne supporter who is trying to whip the party into line behind Washington but has his work cut out for him.

Washington alienated some party regulars during the campaign with his repeated attacks on the city's politically sacred patronage system and his promises to abolish it if elected.

There also is fear in many Democratic strongholds that a black mayor would surround himself with controversial civil rights leaders like the Rev. Jesse Jackson, encourage integration of more white neighborhoods and pay little attention to the needs of whites.

Moreover, supporters of the third candidate in the Democratic primary, Cook County States Attorney Richard M. Daley, son of the late mayor, are avowed archenemies of Byrne but skeptics about Washington.

As late as Tuesday, Byrne had been optimistic, though vague, about her campaign. But her political house of cards was crumbling.

Two of the key persons she announced as top officials of the write-in drive, chief fund-raiser Thomas E. King and campaign manager James W. Bidwell, said they would not work with Byrne.

When candidates filed updated reports of how much money they had raised in contributions of $500 or more, Washington listed $107,000; Epton $82,000, and Byrne $3,000. She had raised $10 million for the primary and spent an estimated $9 million.

Late Tuesday, Byrne's lawyers asked the Chicago Board of Election Commissioners to ease the write-in process by printing the words "For Mayor," and a line for the candidate's name on the envelope encasing ballot cards.

Otherwise her supporters would have had to write the words, draw the line, make a box and check it to indicate their write-in vote, an awkward process that Byrne strategists said would discourage many potential Byrne voters.