Keir Starmer has lashed out at Jeremy Corbyn insisting he was "wrong" on Nato.
Mr Starmer travelled to Brussels for talks with Nato's secretary general Jens Stoltenberg, the first Labour leader to do so in more than a decade.
The Labour leader said he was keen to re-establish the party’s relationship with Nato after his predecessor called for the alliance to be disbanded.
Mr Starmer has said the Labour party's commitment to the alliance is now "unshakeable".
Distancing himself from his predecessor's views, he told the BBC : " Jeremy Corbyn had a very different view, he was wrong about that and I spoke out at the time about that."
Mr Corbyn had described Nato as a "major problem and a major difficulty" before becoming leader of the Labour Party - but never made it party policy to withdraw.
Mr Starmer later launched a fierce attack on the Stop the War coalition, in which Mr Corbyn is a leading figure, accusing it of siding with Russia against Nato.
In an opinion article for the Guardian, he described the group as "naive" at best before adding Putin's regime would see the group as “virtue signallers” who were “providing a smokescreen so it can go on beating up and jailing those brave individuals that dare to stand up to its despotism on the streets of Russia”.
It comes as Boris Johnson warned tensions in Ukraine will reach a “dangerous moment” in the coming days with Russia on the brink of doing something “disastrous”.
The Prime Minister travelled to Brussels and Poland in a show of support to his Nato allies describing the the situation as "one of the worst European crises in decades".
Russia has denied it plans to invade Ukraine but has massed 100,000 troops on the border and has carried out army exercises as it flexes its military muscles.
Standing alongside Nato chief Jens Stoltenberg in Brussels yesterday [THURS], Mr Johnson said: "The stakes are very high and this is a very dangerous moment.
"At stake are the rules that protect every nation - big and small."
But he insisted Britain has an "absolute" and "immovable" commitment to bolstering Europe's security.
Foreign Secretary Liz Truss urged her controversial Russian counterpart Sergei Lavrov to pull troops back from Ukraine's border to ease tensions.
Ms Truss's visit to Moscow was the first by a Foreign Secretary in four years, with the relationship between the UK and Russia severely strained by incidents including the 2018 Salisbury nerve agent attack.
She said: “There is still time for Russia to end its aggression towards Ukraine and pursue the path of diplomacy.
"But Nato is very clear that if that path is not chosen there will be severe consequences for Russia, Ukraine and the whole of Europe.”
But Mr Lavrov rejected Ms Truss’s call for forces to pull back. “The demands to remove the Russian troops from the Russian territory cause regret.
"We don’t want to threaten anyone. It’s us who are facing threats.”
Ms Truss's diplomatic efforts appeared to have failed as Mr Lavrov likened their conversations to "the mute with the deaf" and he accused UK diplomats of coming “unprepared” to their crunch talks.