The offer, to be made in Kazakhstan Tuesday, is expected to have what the five permanent members of the U.N. Security Council and Germany -- known as the P5-plus-1 -- consider "significant new elements," the diplomat told reporters in London on condition of anonymity.
"We will take an offer with us which we believe to be a substantial and serious offer," the diplomat said, offering no details.
CNN cited Western officials as saying the delegates plan to offer eased sanctions currently preventing trade with Iran in gold and other precious metals, if Iran shuts down its underground Fordo uranium-enrichment plant south of Tehran, near the holy city of Qom.
The delegates would also say Iran must get rid of its stockpile of uranium enriched to 20 percent purity, the officials said.
Twenty-percent enrichment puts Iran a few technical steps away from weapons-grade uranium.
Iran has been stockpiling enriched uranium for years in defiance of Security Council resolutions demanding a halt to the activity.
Iran already rejected the new P5-plus-1 proposal, CNN said.
But officials told the network they believed Iran's crumbling economy, due to Western sanctions, may cause the regime to consider making a deal.
The network described the new offer as a slightly revised version of a package P5-plus-1 delegates presented to Iran during talks in Moscow, Baghdad and Istanbul, Turkey, last year.
During those talks Britain, China, France, Russia, the United States and Germany proposed offering fuel for a medical reactor and eased sanctions so Tehran could buy spare parts for its civil aircraft, if the regime suspended its uranium enrichment and shipped its stockpiles out of the country.
"We couldn't come back with the same proposal," one official told the network.
"But the idea is to test the waters and see where the Iranians are and if they are serious," the official said. "We hope to get some insight into their thinking and see what they prioritize in their asks and offers."
Iran insists it is legally entitled to enrich uranium for peaceful purposes and denies it wants to become a nuclear weapons state.
By contrast, the International Atomic Energy Agency says it can no longer verify Iran's nuclear program is strictly peaceful.
Washington and European allies have imposed increasingly onerous sanctions on the country in response to the enrichment program.
Israel has warned it may carry out military strikes to stop Iran from gaining nuclear-weapons capacity. Washington says it won't rule out using force but prefers diplomacy.