Iran revealed the existence of a covert uranium enrichment facility to the U.N. nuclear watchdog this week after it discovered the project's secrecy had been breached by Western intelligence agencies, FOX News has learned.

The U.S., Britain and France will accuse Iran on Friday morning of building the facility underground in secret and charge that it has hidden the facility for years from international weapons inspectors, a senior White House official said.

At an announcement before the opening of the G-20 economic summit in Pittsburgh, President Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy will demand weapons inspectors from the International Atomic Energy Agency be granted immediate access to the facility.

An official told FOX News that Iran revealed the existence of the second plant in a letter sent Monday to IAEA chief Mohamed ElBaradei.

Iran is under three sets of U.N. Security Council sanctions for refusing to freeze enrichment at what had been its single known enrichment plant, which is being monitored by the IAEA.

A senior administration official told FOX News that the U.S. has been tracking the secret project for years, but Obama's announcement comes after Iran discovered in recent weeks that the West had been monitoring the project.

Iran's letter to the U.N. contained no details about the location of the second facility, when — or if — it had started operations or the type and number of centrifuges it was running.

"They [Iran] have cheated three times, and they have now been caught three times," an unnamed official with access to the intelligence told The New York Times.

But one of the officials, who had access to a review of Western intelligence on the issue, said it was about 100 miles southwest of Tehran and was the site of 3,000 centrifuges that could be operational by next year.

Iranian officials had previously acknowledged having only one plant — the one under IAEA monitoring — and had denied allegations of undeclared nuclear activities.

The last IAEA report on Iran in August said Iran had set up more than 8,000 centrifuges to churn out enriched uranium at the cavernous underground Natanz facility, although the report said that only about 4,600 of those were fully active.

The Islamic Republic insists that it has the right to the activity to generate fuel for what it says will be a nationwide chain of nuclear reactors. But because enrichment can make both nuclear fuel and weapons-grade uranium, the international community fears Tehran will use the technology to generate the fissile material used on the tip of nuclear warheads.

The revelation of a secret plan further hinders the chances of progress in scheduled Oct. 1 talks between Iran and six world powers.

At that meeting — the first in more than a year — the five permanent U.N. Security Council members and Germany plan to press Iran to scale back on its enrichment activities. But Tehran has declared that it will not bargain on enrichment.

While Iran's mainstay P-1 centrifuge is a decades-old model based on Chinese technology, it has begun experimenting with state-of-the art prototypes that enrich more quickly and efficiently than its old model.

U.N. officials familiar with the IAEA's attempts to monitor and probe Iran's nuclear activities have previously told the AP that they suspected Iran might be running undeclared enrichment plants.

The existence of a secret Iranian enrichment program built on black-market technology was revealed seven years ago. Since then, the country has continued to expand the program with only a few interruptions as it works toward its aspirations of a 50,000-centrifuge enrichment facility at the southern city of Natanz.

The Associated Press contributed to this report