NEW YORK (AP) _ A judge has delayed the start of the tax evasion trial of billionaire Harry Helmsley, his wife and two former associates until a court-appointed doctor determines if the tycoon is fit to stand trial.

Jury selection had been scheduled to begin Monday, but U.S. District Judge John Walker said Thursday that the issue of Helmsley's health must be resolved first. He said he would appoint a doctor as quickly as possible and did not set a new trial date.

Lawyers for Helmsley claimed earlier this week that the 79-year-old real estate magnate was too ill to stand trial with his wife, Leona, the self- styled ''queen'' of the Helmsley hotel chain, and two former officials of Helmsley companies. They had sought to sever his case from the other three and postpone his trial indefinitely.

Defense attorney John Wing told Walker on Tuesday that of three neurologists and two neuro-psychologists who issued reports, all but one agreed on Helmsley's condition, the nature of which was not disclosed in open court.

Assistant U.S. Attorney James DeVita had asked Walker for a three-week delay to allow a government doctor to evaluate Helmsley.

The judge said he would select an out-of-town doctor who would not be affiliated ''with any institution that may have received any beneficence from Mr. Helmsley.'' He said the doctor would review the other medical reports, interview Helmsley and possibly conduct further tests.

Neither of the Helmsleys was in court.

Helmsley and his 67-year-old wife were indicted in April on state and federal charges that they disguised about $4 million in renovations to their Greenwich, Conn., mansion as business expenses and took bogus tax deductions.

They have pleaded innocent to the 188-count state indictment and the 47- count federal indictment. State prosecutors have agreed to let the federal case go to trial first.

Also Thursday, Walker denied a request by the Helmsleys' co-defendants, Frank J. Turco and Joseph Licari, for a separate trial.

The judge also agreed to let the Helmsleys argue at trial that Turco and Licari allegedly created a phony invoice scheme to steal from the couple rather than to help the Helmsleys defraud the government. A Helmsley attorney said that argument was essential to the couple's defense.