NEW YORK — Honeywell offered more than $90 billion to acquire United Technologies, revealing for the first time to company shareholders the details of a proposed deal to combine the industrial conglomerates.

Honeywell may need the support of investors if United continues to turn away its approaches to create a mammoth, single company.

United CEO Gregory Hayes responded Friday by sending a letter to shareholders, telling them that the proposal ''grossly undervalues'' United Technologies and is not ''an attractive deal.''

United rejected Honeywell's offer, saying that a tie-up would face tough regulatory obstacles due to antitrust issues. Both of the companies are big players in the aerospace sector.