Jeremy Corbyn vowed to lead Britain out of the EU as he insisted Labour is "ready any time" for another general election.
The Labour leader said his party would "absolutely" ensure Brexit occurs if they secured power, with a focus on negotiating tariff-free access as part of a "jobs-first Brexit".
Labour won 262 seats in the General Election, up from the 232 secured by Ed Miliband in 2015, but the Conservatives remain the largest party in Parliament.
Mr Corbyn believes that without an outright majority Mrs May's position is vulnerable and he intends to oppose the Queen's Speech and table a "substantial amendment" in an attempt to bring down her administration.
Another general election within months is also expected, Mr Corbyn said.
Speaking on the BBC's Andrew Marr show, Mr Corbyn said: "We're going to put down a substantial amendment to the Queen's Speech which will contain within it the main points of our manifesto, and so we'll invite the House to consider all the issues we put forward which I've mentioned - jobs-first Brexit, mention the issues of young people and austerity, there's many other things."
On Brexit, Mr Corbyn said he wants a "jobs-first Brexit" negotiated as quickly as possible along with guaranteeing the post-Brexit rights of EU nationals living in the UK.
Asked if Labour would seek to keep Britain in the European single market and customs union, Mr Corbyn said: "Well, the single market is a requirement of EU membership and since we won't be EU members there will have to be an arrangement made."
Pressed if he was being clear that the UK will leave the EU, Mr Corbyn said: "Absolutely. Where I frame it is, we want a tariff-free access to the European market, we also want to maintain a very important university and research collaboration in Europe, and there's a whole host of European agencies - Euratom, security, environment - in which we wish to be part of.
"I tell you this, we will absolutely remain part of the European Convention on Human Rights and European Court of Human Rights - we are not walking away from those vital post-war agreements that were made."
On the so-called "Great Repeal Bill," designed to transfer EU law into British law to enable changes, Mr Corbyn said: "The Great Repeal Bill, I suspect, has now become history.
"I suspect we'll have something different coming in a couple of weeks time. We will put forward a position in which we negotiate tariff-free access to the European market and legislate after that."
Asked if there will be another general election this year, Mr Corbyn said: "I think it's quite possible there'll be an election later this year or early next year, and that might be a good thing because we cannot go on with a period of great instability.
"We have a programme, we have support and we're ready to fight another election campaign as soon as may be, because we want to be able to serve the people of this country on the agenda we put forward, which is transformative and has gained amazing levels of support."
Mr Corbyn said the party raised millions of pounds from an average donation of £22 per person as he sought to reassure that Labour has the funds in place for another campaign.
He added: "We're ready any time."
Questioned if he is in it for the long-term, Mr Corbyn replied: "Look at me, I've got youth on my side."
Mr Corbyn had earlier told the Sunday Mirror "I can still be prime minister" and that he expects to be able to attract some of the party's biggest names to serve on his front bench.
"My phone is full of texts from lots and lots of people from right across the party," he told the newspaper.
"I'm very happy about that. I'm very proud to lead this party. And I'm open to everyone. It's important to make that clear."
Mr Corbyn said he does not expect Sinn Fein's seven MPs to take their seats.
Such a decision, which would be in keeping with Sinn Fein's previous stance, would reduce the number of MPs the Tories need to have a working majority.
Mr Corbyn said: "I don't see any possibility of Sinn Fein taking their seats."
Shadow chancellor John McDonnell said he believes there is a majority in Parliament to maintain the winter fuel allowance and triple lock pension, two things the Conservatives proposed changing in their manifesto.
He added to ITV's Peston on Sunday: "I believe the DUP is in favour of scrapping the bedroom tax. There's a whole range of issues like that where we think there'll be a majority in Parliament."
Mr McDonnell said Labour should "draw on upon all the talents" when it considers its new frontbench although he also gave praise to the existing shadow cabinet.
He said: "Our shadow cabinet at the moment was a winning team. It's just won, effectively, votes that no one predicted we would so I don't want to break up that winning team."
Mr McDonnell said Mr Corbyn would decide the shadow cabinet positions, reiterating "It's a winning team I think we should hold together", adding there are also other positions to be filled.