A career FBI agent, described as the bureau's top expert in court-ordered secret entries and safecracking, has been under federal investigation for more than two years on allegations of tax evasion and other offenses, the agent's lawyer said yesterday.

The agent, H. Edward Tickel, 42, came under FBI investigation in April 1980 -- while he was head of security at bureau headquarters in Washington -- in connection with an alleged entry at the FBI's credit union office, according to Tickel's attorney, Kenneth Michael Robinson. Robinson said Tickel, acting on an anonymous telephone tip, had gone to the office and found a door unlocked and a safe open.

In a spinoff from that investigation, Robinson said, federal agents began to look at Tickel's financial records, particularly the purchase of a $10,000 boat, the purchase of equipment from a Florida gun shop, and Tickel's sale of a $37,000 diamond. Robinson said that while federal agents may have suspected that the diamond had been stolen and that Tickel was acting as a middleman or "fence," subsequent investigation showed that the gem was not stolen. No charges have been brought against Tickel in any of these matters.

Tickel, in an interview in Robinson's office, denied all the allegations against him. "I've worked hard for the bureau. I've been extremely loyal and I have not done anything wrong," Tickel said.

Federal officials have refused to discuss the investigation. However, in a news conference yesterday, Acting Attorney General Rudolph W. Giuliani vigorously denied a report by Scripps-Howard Newspapers that the FBI had tried to fire Tickel instead of investigating him.

Giuliani called those reports "false and outrageous" and said that the FBI promptly had reported allegations involving Tickel to the Justice Department and the U.S. attorney's office.

In a statement, Giuliani said "the employe has not been fired by the FBI because at this time there is not enough evidence to warrant discharge."

Tickel, a 14-year veteran of the FBI and the son of a career agent, had been with the bureau's technical services division, which installs secret, court-ordered microphones and other electronic surveillance equipment, until November 1981.

At that time, Robinson said, Tickel -- who agrees that nobody tried to fire him -- was transferred to the FBI's procurement and property management section and since then has not participated in any surveillance work.

Yesterday, Giuliani said that Tickel, whom Giuliani did not identify by name, has been reassigned to a unsensitive position pending resolution of all allegations.