Cambridge officials will appear in court today to ask for a second postponement of a ruling on the legality a citywide ban preventing nerve gas testing. A postponement would give the city time to conduct an investigation into the safety of the tests.

The Middlesex Superior Court had temporarily blocked the ban at the request of the Arthur D. Little Company (ADL), a local research firm investigating toxic chemicals for the military at a new laboratory in North Cambridge.

Prompted by concern over the safety of the laboratory facility and 5 possible health risks to neighborhood residents, the Cambridge Department of Public Health issued the ban last month.

ADL protested the move on the grounds that to obey it the company would be forced to violate laws governing treatment of federal government property.

Company and city officials faced each other in court last week, when the first temporary restraining order on the ban ran out. At the hearing, ADL presented an affidavit from the company's safety office, as well as letters from members of the Department of Defense, attesting to the safety of the facility, City Solicitor Russell B. Higley said yesterday.

Judge Joseph Mitchell granted a one-week extension of the injunction to give the city time to refute the company claims.

But Higley said the seven-day period was not long enough and that he will request a further extension today. The city last week hired an outside firm, TRC Environmental Consultants of Hartford, Connecticut, to investigate lab security and safety. The Cambridge City Council also recently launched its own scientific advisory committee to make recommendaions on technical matters, although neither group has had time to complete its inquiries.

Higley said he was confident the court would grant the extension.

ADL spokesman Alma Triner said yesterday she would be "very surprised" if the city could ever prove the lab was unsafe, adding that "ADL is one of the safest labs in the world."