Britain is monitoring a Russian plane and three British aircraft in an ever widening probe into the death from radiation poisoning of a former Russian spy that has heightened tensions with Moscow. Other planes may need to be tracked, Home Secretary John Reid told parliament on Thursday (November 30) as he pledged there would be no political barriers to the probe. British Airways (BA) said three aircraft had been taken out of service as part of the probe. All had flown between Moscow and London, and one is still in Moscow. The airline said "very low traces" of a radioactive substance had been found on the two planes being held in London. A fourth plane being monitored, a Boeing 737 leased by Russian carrier Transaero that landed at London's Heathrow Airport on Thursday, was given the all clear. Many passengers travelling to Moscow on British Airways from Heathrow on Thursday said they felt there was no risk in travelling. "They've assured me that that's all fine. So I'm not going to think about that when I am on the flight," said one woman bound for the Russian capital. Another couple said: "We're confident that BA wouldn't let us fly if we, they have already scanned the planes they've found everything that was there and it has been cleaned up and it was such small amount anyway,". The Kremlin and Russia's foreign spy service have denied any involvement in the death of Alexander Litvinenko, a former Russian spy who became an outspoken critic of Russian President Vladimir Putin. Litvinenko, who became a British citizen while living in London, died a week ago after being poisoned with radioactive polonium 210. Reid said police had found radioactive traces at 12 out of 24 locations being checked in connection his death. Litvinenko accused Putin of ordering his death but the Kremlin and Russia's foreign spy service have denied any involvement. The announcement about the planes and their destinations could rekindle suspicions of a Moscow link to the poisoning. Alex Goldfarb, a close friend of Litvinenko, told reporters in London he suspected the radiation had come from Moscow. "This kind of reinforces the theory that the origin of this material that killed Alexander is in Moscow," Goldfarb said as an inquest into Litvinenko's death was opened in London and promptly adjourned. Reid told parliament the inquest was suspended pending further scientific research to be carried out by police. He said a post mortem will be carried out on Friday (December 1). Reid told parliament that Moscow had promised cooperation to the "highest level" and that British police would use all the powers they needed to search planes. "There certainly will be no political prohibition on the police following where the evidence leads them," he said. In Britain, thousands of BA passengers sought health reassurances from the airline after the announcement. BA says the risk to health is low. But it faces a huge task tracing the 33,000 passengers who used the planes over a five-week period. In Russia, the Transport Ministry ordered international airports to step up security controls on foreign carriers. Reid said Britain would contact the governments of every country where the planes may have landed.