Labour has pledged to ban all zero-hours contracts, put a halt to unpaid internships and end the pay cap on public sector staff in an unashamedly leftwing pitch to British workers.

In a move welcomed by union leaders but that will be attacked as “anti-business” and unaffordable by opponents, the 20-point blueprint also includes commitments to double paid paternity leave to four weeks, increase paternity pay and guarantee temporary and part-time workers the same rights as full-time employees.

The shadow chancellor John McDonnell said the policies would form “the cornerstone of the next Labour government’s programme to bring an end to the rigged economy that many experience in workplaces across Britain”. The package includes promises to increase the minimum wage to match the national living wage (expected to be at least £10 an hour by 2020) and to introduce a maximum 20:1 pay ratio between the highest and lowest earners in the public sector and in companies bidding for public sector contracts.

McDonnell said: “The scandal of six million people earning less than the living wage and four million children growing up in poverty are not inevitable. It only takes a change of government to bring these outrages to an end.”

An Opinium/Observer poll puts Labour a massive 17 points behind the Tories, on 30%, but its support has increased by four points compared with a week ago. Backing for the Tories has risen by two points to 47%, with the Lib Dems down three points to 8% and Ukip down two points to 7%.

Labour MPs will hope the appeal to working people will stem what many feared would become a drift of some traditional Labour supporters towards the Tories with less than six weeks to go before polling day on 8 June.

The manifesto for working Britain is more leftwing than that unveiled by Ed Miliband ahead of the 2015 election, which itself was criticised by the Tories and Lib Dems at the time as one of the most “anti-business” in living memory.

The 20-point blueprint aims to offer concrete measures to help those who Jeremy Corbyn says are being exploited in an increasingly unregulated economy, including workers such as couriers and delivery drivers, and the 900,000 people employed by the big high street brands and other companies on zero-hours contracts.

Len McCluskey, general secretary of the UK’s biggest union, Unite, said: “This positive platform from Labour is the antidote to the uncertainty and insecurity which plagues working life for millions in the UK.”

Unison’s general secretary Dave Prentis said: “Ending the 1% pay cap would make such a huge difference to the lives of thousands of nurses, teaching assistants, town hall staff and other public service employees whose living standards have plummeted under the Conservatives. Calling time on zero-hours contracts would come as a huge relief to the many people who have no option but to work in this precarious and wholly unsatisfactory way.”

The Communication Workers Union described the plans as “some of the most important announcements from any political party on the world of work for a generation”.

Labour has already said it will introduce four new bank holidays. McDonnell also signalled that Labour’s manifesto will pledge no rise in VAT or national insurance and he will argue the party should rule out reviving Miliband’s mansion tax policy.

The Tories will use the pledges to further question Labour’s economic credibility and ask how they can be paid for without huge tax rises or a massive increase in borrowing.

On Saturday Corbyn broke off from a campaign speech to speak about his personal history, motivations and views on leadership. In a tactic reminiscent of that employed by the former Labour leader Neil Kinnock and the former Tory prime minister John Major, Corbyn said: “And now for a sentence I’ve yet to utter in my political life: ‘Enough about you, what about me?’” He recalled his maiden speech in parliament 34 years ago, the part he played in anti-apartheid protests in London and other personal memories.

Afterwards he denied it was part of a rebranding exercise but was a “one-off”. “As you know, I am very averse to talking about myself … I am not going to spend the whole election campaign talking about myself any more. So you had the chance today,” he said.

Labour strategists are keen to contrast what they portray as a cold Tory prime minister, Theresa May, with what they insist is Corbyn’s warmer personality. They are also anxious to contrast his willingness to take questions from the media with May’s refusal to take part in TV debates.