Russia will begin to load fuel into the reactor at Iran's Bushehr nuclear power station next week, marking the start of its launch after years of delay, Moscow said today.

Russian and Iranian specialists are to begin loading uranium-packed fuel rods into the reactor on 21 August, a process that will take about two to three weeks. This will be a key step towards starting up the reactor, although it will not be considered operational from that date.

"This will be an irreversible step," Sergei Novikov, a spokesman for Russia's state nuclear corporation, Rosatom, said. "At that moment, the Bushehr nuclear power plant will be certified as a nuclear energy installation."

Novikov said that the head of Rosatom, Sergei Kiriyenko, would travel to Bushehr, in southern Iran, for next week's ceremony. Iran's vice-president, Ali Akbar Salehi, who also heads the Atomic Energy Organisation of Iran, is scheduled to attend as well.

In March, the US secretary of state, Hillary Clinton, on a visit to Moscow, criticised Russia's plans to start up the Bushehr plant, describing them as premature, given western suspicions about Tehran's nuclear programme.

In June, the UN security council approved a fourth round of sanctions against Iran over its nuclear programme, including tougher financial controls and an expanded arms embargo, as well as an asset ban on three dozen companies and a travel freeze on individuals.

Despite American reservations about the facility, nuclear experts say Bushehr does not contain sensitive technology, which is why it does not figure in any UN security council resolutions. Moreover, Bushehr has no link with Iran's secretive uranium enrichment programme, seen as the main "weaponisation" threat, at other installations.

"The Iranians have been able to go ahead with Bushehr because it's clean," said a nuclear expert, adding that the light-water reactor in Bushehr was internationally tolerated because of Russia's involvement.

The Foreign Office said that Britain respected Iran's right to a peaceful civilian nuclear programme as long as it meets its international obligations.

"The Bushehr reactor, which will use fuel provided by Russia, is separate from Iran's proliferation sensitive nuclear activities that are a clear contravention of six UN security council resolutions," the Foreign Office said in a statement. "Today's announcement underlines the fact that Iran does not need to pursue these other activities to enjoy the benefits of nuclear power."

Moscow points out that the project has been closely supervised by the UN nuclear watchdog, the International Atomic Energy Agency, and that Iran has signed a pledge to ship all the spent uranium fuel from Bushehr back to Russia for reprocessing, ruling out the possibility that any of it could used to make nuclear weapons.

Construction of this, Iran's first nuclear plant, was begun in 1975 by several German companies. They pulled out following a US embargo on high-technology supplies to Iran, after the 1979 Islamic revolution and the subsequent US embassy siege in Tehran.

Russia stepped in and agreed to build the 1,000-megawatt reactor 15 years ago, in a project that has been likened to efforts to fix a German engine in a Russian car.

Delays have plagued the $1bn (£640m) project, with diplomats saying that Moscow has used it as a lever in relations with Tehran. Iran has had to put up with the long timescale because it has no other potential nuclear partners.