Vladimir Putin is a "ruthless" dictator whose days are numbered, David Miliband said today as Russians go to the polls.
The former foreign secretary warned it would be wrong to underestimate the "intelligent" leader but predicted he
will not survive a six-year term at the Kremlin.
Mr Miliband chose to make the robust attack in the second edition of The Sun on Sunday, a move that will be a shot across the bows of Labour figures who are trying to distance themselves from the News International stable.
In an article for the newspaper, he wrote: "Whether or not Vladimir Putin wins today, he will not be celebrating a fourth term in office six years from now.
"Whoever wins the election today, one thing is clear: Russia will not be the same.
"The people of Russia have spoken up, and a wise leader would listen."
Mr Putin is expected to return to the Kremlin for his third term as president following one term as prime minister but under an atmosphere of strong public unrest.
His United Russia party barely retained its parliamentary majority after elections staged in December despite alleged vote-rigging.
Police are braced for public disorder once the vote closes following unprecedented mass protests over Mr Putin's system of so-called "managed democracy".
Mr Miliband wrote: "Russian nationalists, communists and liberals who have taken to the streets in their tens of thousands in temperatures well below freezing don't agree about much, but they are united against corruption, stagnation and arbitrary rule.
"It is wrong to underestimate Putin. He is intelligent, worldly and ruthless. In the first term of his presidency, in the wake of the embarrassing latter years of Boris Yeltsin, the rhetoric and to some extent reality was about reform as well as order.
"Russians got their pride back - floating on a tide of oil and gas revenues. But since then Russian reform has gone into reverse, and vested interests consolidated their positions."
Relations between Britain and Russia reached breaking point after the murder in London of dissident critic Alexander Litvinenko.
Correction - An earlier headline on this story incorrectly attributed these comments to Labour leader Ed Miliband, rather than his brother David. We're not the first to have done it, and we probably won't be the last. Sorry.