The leaders of the US, UK and France demanded UN inspectors be given immediate access to the facility.
Iranian officials in New York and Tehran denied the plant was a secret.
Tehran has previously acknowledged it has one enrichment plant, at Natanz.
Iran’s decision to build a secret facility represented a “direct challenge to the basic compact” of the global non-proliferation regime, US President Barack Obama said, making a statement in Pittsburgh, where he is hosting a G20 summit.
Despite Iran’s assertions that the facility was for peaceful purposes, the new plant was “not consistent” with that goal, the US president said.
Iran now faces the prospect of tougher international sanctions if it does not satisfy Western calls for full disclosure in the coming months.
‘Line in the sand’
Tehran would be held accountable for any failure to meet these responsibilities, he said.
Speaking after Mr Obama, the French and British leaders used strong language to insist that Iran would now have to disclose full details of its entire nuclear programme or face new and tougher sanctions.
Gordon Brown stressed that the US, France and UK were “at one” on the issue, and accused the Iranians of “serial deception”.
There was now “no choice but to draw a line in the sand” over the nuclear issue, he said.
Mr Sarkozy said the situation was a challenge to the entire international community.
“Everything must be put on the table,” the French president said, adding that the world needed to see a “step change” from Iran in the coming months.
Iranian officials were quick to deny the latest plant was any kind of clandestine project.
“This installation is not a secret one, which is why we announced its existence to the IAEA [International Atomic Energy Agency],” Ali Akbar Saleri, head of Iran’s nuclear agency, told the AFP news agency.
The existence of Iran’s first enrichment plant, at Natanz, was only confirmed after intelligence emerged from Iranian exile groups several years ago.
Western governments are said to have known of the existence of the new enrichment plant for several months.
It is said to be an underground facility at a mountain on the site of a former missile site belonging to the Iranian Revolutionary Guards north-east of the holy city of Qom.
Construction on the facility started in earnest in mid-2006, diplomatic sources said.
Iran’s letter to the UN watchdog, the IAEA, came as the New York Times cited US officials giving the first details of the new plant.
The IAEA confirmed it received a letter from Iran on Monday informing it that “a new pilot fuel enrichment plant is under construction”.
In response, the IAEA has requested Iran to “provide specific information and access to the facility as soon as possible”, an IAEA statement adds.
The disclosure of the new plant comes one day after world leaders stressed the need for greater co-operation against nuclear proliferation and shortly before Iran is due to resume talks with international powers on the issue.
Reports that Western officials have known of the existence of the enrichment plant for several months suggest that Mr Obama’s policy was put in place even as he was aware of the new construction near Qom, correspondents note.
The talks are due to be held in Geneva on 1 October with Tehran and the five permanent UN Security Council members – US, UK, Russia, China and France – plus Germany.
Iran now faces the prospect of tougher sanctions if it fails to meet international demands for transparency.