Doctors fed up with their pay are quitting the NHS to work abroad - so the UK is effectively training doctors for other countries like Australia and Canada, a union has warned.
The British Medical Association (BMA) - which is leading the industrial action - says the erosion of pay over the least 15 years has led to an exodus of doctors.
At the same time, the government is offering a 6% pay rise, after the Prime Minister accepted recommendations from pay review bodies.
Oz Ly, a BMA member representing doctors in East Anglia, said: "Over the last 15 years, pay has been massively eroded to the point where we are now training doctors for countries such as Australia and Canada.
"Only yesterday, a very capable doctor and member of ours who worked at the Colchester Hospital, I asked her the question, what her next rotation will be.
"Guess what her response was? She's going to Australia.
"And this is the third doctor that I know that I've lost over the last two years."
He continued: "They are angry because of the lack of service patients have been receiving over the last few years.
"There isn't a single hospital that's got their rotas full and one of the reasons is because we are losing doctors to places like Australia, or doctors are completely leaving the profession to go to other far-rewarding careers in terms of salary.
"So the anger is the fact that you've trained to be able to do the job to a certain standard, but you are not being given the tools to be able to do that job."
He warned of an existential crisis for the NHS.
"My worry is, if we don't do something about pay and reward, we're not going to have an NHS to rely on.
"You can't have an NHS without doctors and nurses."
At a picket line outside Addenbrooke's Hospital, they were joined by Australian Labor MP Sarah Andrews.
"Conditions here in Cambridge and across the UK in the NHS are just dreadful.
"The fact is that junior doctors are paid £14 an hour with such a high university debt.
"It's remarkable that they're expected to do the incredibly important work that's expected on them on such a low, disrespectful level of pay."
Junior doctors say want to restore pay to levels 15 years ago.
But Rishi Sunak challenged the BMA to call off strikes after making the 6% pay rise offer.
He said: “The Government has not only made today’s decision on pay. We’ve backed the NHS with record funding, delivered the first ever, fully funded long-term workforce plan and met the BMA’s number one ask of Government, with a pensions tax cut worth £1 billion.
“So, we should all ask ourselves, whether union leaders – or indeed political leaders – how can it be right to continue disruptive industrial action?
“Not least because these strikes lead to tens of thousands of appointments being cancelled – every single day and waiting lists going up, not down.”
The government has also accepted pay recommendations from independent review bodies for other public sector workers.
The current level of CPI inflation is at 8.7% and Mr Sunak says he wants to avoid pay rises that could fuel a wage-price spiral.
In a direct message to public sector unions, Mr Sunak said: “Today’s offer is final. There will be no more talks on pay.
“We will not negotiate again on this year’s settlements and no amount of strikes will change our decision.”
But with no new borrowing, Mr Sunak said government departments will have to “reprioritise” spending.
Want a quick and expert briefing on the biggest news stories? Listen to our latest podcasts to find out What You Need To Know