Sir Keir Starmer has not ruled out launching nuclear weapons to protect the UK if he becomes Prime Minister next year on a visit to British troops in Estonia near the border with Russia.
The Labour leader also said that he would consider setting up off shore processing centres to deal with the surge in small boat crossings.
Sir Keir also backed the Irish government's use of European human rights laws to challenge a proposed amnesty for British soldiers during the Troubles in Northern Ireland.
And Sir Keir said he was ready for a spring general election in as little as 19 weeks’ time, but admitted he was worried about the impact on his wife and two children if he becomes Prime Minister..
Keir Starmer speaking to GB News Politics Editor Christopher Hope
GB News
Visiting British troops at a Nato base in Tapa, Estonia on Thursday, fewer than 100 miles from the Russian border, Sir Keir made clear that he would be willing to fire nuclear weapons to defend the UK, in stark contrast to his predecessor Jeremy Corbyn who said he would not use them.
Asked directly if he would fire nuclear weapons to protect the UK, Sir Keir said: "The nuclear deterrent is very important to us.
“Obviously, I'm not going to discuss with you the circumstance in which they may or may not be used, but it is a very important part of the defence of our country that we are strongly committed to, along with our commitment to NATO and to the security of our nation, which is paramount."
Sir Keir - who last visited British troops at the NATO base in March last year - made clear that the UK's commitment to NATO will not change if a Labour government is elected.
He said: "There's been an unshakable commitment to NATO ever since NATO was first created, which of course was under a Labour government. ...There's a real driving sense of purpose here with the troops that are here on the frontline."
Sir Keir said that he was keen to increase defence spending from the current level of 2.12 per cent of gross domestic product, perhaps to the 2.5 per cent level it achieved when Labour lost power in 2010.
He said: "The ambition is for an increase. Well, when the last Labour government was in power, it was 2.5%. But you know, there is a NATO commitment here, which we will always make sure that we maintain."
The general election is now just 19 weeks away if it is held at the same time as May's local elections. Sir Keir said he and his team were ready, with a manifesto ready "when we need it" if a snap poll is called.
He said: "We are ready for a general election. I've had my whole team on a general election footing for some time now.
"Given the complete state of failure now in the country, there's a real sense that everything is broken, nothing is working.
"The sooner that election comes the better, because for millions of people they can't afford to wait any longer for that general election....
"The sooner that is called, the better. We're ready for it."
Sir Keir admitted he was concerned about the impact on his wife Victoria and his two teenage children about the scrutiny that comes with running to be Prime Minister.
He said: "I do worry about my family. If I'm honest, I'm very protective of my wife and our children.
"We don't name our children publicly. We don't have photo shoots with them. And so I am very mindful of the impact it could have on them.
"Our boy is 15, our girl is 13. These are, you know, ages where they're going through huge change. But my aim is to keep protecting them in every way that I can."
Elsewhere Sir Keir said Labour would look at setting up offshore processing centres to deal with illegally-arrived migrants, rather than send them to Rwanda, as the Tory government intends.
He said: "There's a difference between offshore processing and essentially deporting people, which is the Rwanda scheme. The Rwanda scheme is a gimmick, a very, very expensive gimmick.
"We've paid hundreds of millions of pounds and nobody has gone to Rwanda.
"Other countries do offshore processing and that is something, you know, I'll look at any credible option, but I think that at the moment, the single most important thing is to take out the gangs that are taking money to put people in those boats."
On taxation Sir Keri said that his Government would seek "to reduce the tax burden but the key to this is economic growth".
Sir Keir also made clear that he backed Ireland's challenge to a UK law which offer immunity to UK soldiers who served in Northern Ireland during the Troubles.
He said: "I think the Government's got this one wrong... To press on with this proposal when there's no political support in Northern Ireland from any political party and most importantly when the victims and their families in Northern Ireland are not supportive, is in principle wrong for me. So that is why we don't support this proposal, this legislation coming from the government."
Asked if Sir Keir - a keen Arsenal FC fan - would rather a general election win for Labour or for the Gunners to be crowned premier league champions, he said: "Of course I've got to go for the election victory and hope that [Arsenal manager Mikel] Arteta does the other bit for me."
