United Airlines said yesterday that it plans to cancel or "substantially delay" delivery of 20 of the 39 new-generation Boeing 767 aircraft it has on order.

The 20 jetliners, including spare parts, are valued at about $1 billion.

United Chairman Richard J. Ferris warned that United might cancel more of its 767 order if Congress repeals or significantly modifies controversial provisions of the 1981 tax bill that allow corporations to buy and sell tax breaks.

In a speech to the Aero Club yesterday, Ferris cited the sluggish economy, flight cutbacks caused by the air traffic controllers' strike, uncertainty about the tax-leasing provisions and "the need to be financially prudent" as reasons for its decision to cancel or delay delivery of the 767s.

United, the nation's largest carrier, kicked off Boeing's 767 program almost four years ago when it ordered 30 of the planes, boosted since to 39. Delivery of its first plane is scheduled for August.

The order was placed at a time when the airline industry--and United--were experiencing record profits. Last year, the airline reported a loss of $104.4 million, which was followed by a first-quarter loss of $129.3 million this year, its largest quarterly loss ever.

Boeing declined yesterday to assess the impact of the United cancellation, issuing a statement that acknowledged United had asked to begin negotiations to stretch out deliveries or cancel some of the orders.

However, aviation sources say the impact could be significant, at least in the near-term until the airline industry is in better financial health and interest rates have declined. Although Boeing has 173 firm orders for the plane--from six domestic and 11 foreign airlines--no new orders have been placed since at least last summer. Until now, no airline has canceled any 767 orders although American Airlines recently canceled 15 of the Boeing 757 aircraft it had ordered.

The 767, a twin-engine, wide-body jetliner seating about 200, is the first entirely new Boeing commercial passenger aircraft to go into production since the 747 in 1966. The cost of the plane is about $40 million.