Tony Blair has declared that no "diplomatic or political barrier" would be allowed to stand in the way of the investigation into the death of former Russian spy Alexander Litvinenko.
Mr Blair said the death of Mr Litvinenko, who apparently died after ingesting radioactive polonium 210, was being treated as a "very, very serious matter".
He said the authorities were determined to get to the bottom of what had happened.
Mr Blair was speaking following a joint news conference with Danish prime minister Anders Fogh Rasmussen in Copenhagen during a stop-off on his way to the Nato summit in the Latvian capital of Riga.
Mr Blair said he had not yet spoken to Russian president Vladimir Putin, who is also expected in Riga although not until Mr Blair has left and the summit is over.
He said that he would do so when the time was "appropriate".
He added: "There is no diplomatic or political barrier in the way of that investigation going where it needs to go.
"It is obviously a very, very serious matter indeed. We are determined to find out what happened and who is responsible."
He said the Government had already made clear that it expected full cooperation from the Russian authorities in the investigation.
Mr Putin has strongly denied any Kremlin involvement in Mr Litvinenko's death.
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