John has urged nuclear test veterans and their families to contact him as he vowed to fight for justice.
Around 20,000 UK forces personnel were posted at nuclear test sites in Australia and the South Pacific, from the 1950s.
From the early 1980s, the veterans began to report rare cancers, sterility, their wives having miscarriages and birth defects in their children.
John, who is Labour’s shadow defence secretary as well MP for Wentworth & Dearne, recently met some of the veterans and their families, along with Labour leader Keir Starmer.
He said: “Their campaign is our campaign. We want the Government to recognise the debt our country owes them.
“Our job now is to get the government to listen, get Boris Johnson to sit down with veterans and their descendants, look them in the eye, listen to their experience, and make his own judgement as we have about the recognition we think is long overdue.
“I’ve set a deadline of the first test’s 70th anniversary – October 2022 – for us to work to, and I hope by then the Prime Minister will do the right thing and give them a medal.”
The veterans previously challenged Boris Johnson to “look me in the eye” and explain why they were refused a medal last year.
At their meeting, Sir Keir thanked the veterans for their service and told them: “The country owes you a debt of honour. The terrible decision to not award a medal was morally completely wrong. The Prime Minister must now meet you, hear what you have to say, and do what it takes. We will support you all the way.”
Forces personnel maintained the test sites, took samples, ran laboratories, did decontamination and transported and disposed of radioactive material.
They slept in camps as close as five miles from the blast zones and often only wore shorts and boots.
Over the last decade the evidence has become clearer yet the UK remains the only nuclear power that refuses them recognition or compensation, unlike the US, France, Canada and Australia.
There are 155,000 descendants yet their stories are largely untold.
John added: “I know that Forces veterans can be reluctant to speak up about their experiences, but I’d like to appeal to them to contact me directly.”