Lives will be saved. Hundreds of them, every year. That’s the first and most important thing to say about the organ donation (deemed consent) bill, which is expected to clear parliament today and become English law in 2020.

You may already know it as Max and Keira’s Law, after the young boy and girl whose story inspired politicians of all parties to get behind a change that will take us from having to opt in as an organ donor to having to opt out. The government expects that as many as 700 lives a year may be saved – as Max Johnson’s was.

Two years ago, he was lying in a hospital bed in Newcastle, being kept alive by machines. Max was nine years old. His heart had failed after an infection and the only thing that could save him was a transplant. To put that in context, there are 6,000 people on the UK transplant waiting list, and around 400 a year die waiting.

At the same time as Max lay dying, Keira Ball was on her way to a free-running event in the West Country with her mum, Loanna, and her brother. She was also nine years old. Tragically, their Vauxhall Vectra was in a head-on collision with a 4×4 driven by a farmer and towing a cattle trailer. Keira was rushed to Bristol Royal hospital for children, where it was found she had a severe brain injury.

There was nothing the doctors could do. They asked her father, Joe, if he would allow her organs to be used to save others. That was a terrible moment for all concerned, but Joe later said: “Although we’d never discussed organ donation, I knew it was what Keira would have wanted. It was in her nature.”

I have heard the same from many families I have met through a charity called the Donor Family Network, which offers them support. They all say it is much better to have a conversation about donation in a happier time, instead of at a moment of crisis. They also all find comfort in celebrating organ donation as the gift of life. We will need to find a way to go on doing that, even when it is assumed in law that everyone is willing.

There are other obstacles to be overcome, too, if the new system is to work. Mistrust still lingers for some after the scandal of a generation ago at the Alder Hey children’s hospital, where the hearts and organs of babies were stored or used for research without the knowledge or consent of parents. Laws have been changed and lots more has been done to remove the arrogance and secrecy of the past, but it must not be allowed to return under a new system that assumes the right to take our body parts.

I was squeamish about all this until I became personally involved after writing and presenting the BBC Radio 4 series The Boy Who Gave His Heart Away and writing the book of the same name. Then I met the families of Martin Burton and Marc McCay, teenage boys of 15 and 16 who became ill at the same time. They did not know each other, but their fates became inextricably bound when Martin died and his heart saved Marc’s life. Years later, I saw Martin’s mum, Sue, reach out and touch Marc’s chest and feel the heart of her son, still beating away.

Much the same happened when Keira’s father met Max last year, only he had brought a stethoscope to listen. It was emotional, of course. Max said: “They saved my life. It’s hard to just say thank you. You feel like it’s not enough.” The new legislation was going to be called Max’s Law, but he asked for Keira’s name to be added.

The law is due to change in England in April 2020. From then on it will be assumed that everyone is willing to be an organ donor after death, unless they have said otherwise. At the moment, 80% of adults in England say they would definitely donate their organs or would consider it. But only 37% have signed up as donors. Those who want to opt out for personal, religious or other reasons will still be able to. It’s important we take that seriously. Some people will be excluded, including children and tourists.

Will it work? I’m no expert, but Wales made a similar change in 2015 and it seems to be working. The number of donors in Wales climbed from 61 in 2016-17 to 74 the following year, and was already up to 72 by three-quarters of the way through 2018-19, according to NHS Blood and Transplant.

Spain is held up as the biggest example of what an opt-out system can do, with 46.9 donors per million people as opposed to 24.4 per million in England. But Spain has invested heavily and, as Sue, says: “Do we have the infrastructure, retrieval teams, surgeons, specialist nurses and ICU beds that would be needed?” Not yet, perhaps. So there are reasons to be cautious – about the way patients are treated and whether we still see donation as a gift – and there is one huge caveat: the NHS needs the money to cope with this. But the news today means change is coming and there is reason to celebrate, because that change will save lives.