Three former high-ranking American military officers have warned against any military attack on Iran.
Gen Hoar was one of the signatories of the letter
They said such action would have "disastrous consequences" for security in the Middle East and also for coalition forces in Iraq.
They said the crisis over Tehran's nuclear programme must be resolved through diplomacy, urging Washington to start direct talks with Iran.
The letter was published in Britain's Sunday Times newspaper.
It was signed by:
- Lt Gen Robert Gard, a former military assistant to the US defence secretary
- Gen Joseph Hoar, a former commander-in-chief, US Central Command
- Vice Adm Jack Shanahan, a former director of the Center for Defense Information
"As former US military leaders, we strongly caution against the use of military force against Iran," the authors said.
Iran's account of its nuclear programme has not satisfied the US
They said such action would further exacerbate regional and global tensions.
"A strategy of diplomatic engagement with Iran would serve the interests of the US and the UK and potentially could enhance regional and international security," the letter said.
It also said that "the British government has a vital role play in securing a renewed diplomatic push and making it clear that it will oppose any recourse to military force".
The US and its Western allies suspect Iran of using its nuclear programme as a cover to produce nuclear weapons, a claim denied by Tehran.
Washington has so far refused to rule out military action if Iran does not halt its nuclear activities.
The US has also recently beefed up its military presence in the Gulf.