Tom Jackson was speaking after the British Medical Association (BMA) announced it would begin a five day strike later this month as part of an ongoing disagreement with the government over the contracts, which are being forced upon them.
Junior doctors have already embarked on industrial action in the row, including withdrawing emergency care, but have not as yet gone on strike for five days at once.
The BMA originally accepted a contract offer from the government in May but when this was taken to a member's vote it was rejected.
The proposed five-day walkout will begin on September 12th.
The NHS is making contingency plans as it's expected one million hospital appointments may have to cancelled and a thousand operations cancelled.
The old contract:
Weekend and night work earns an extra 20-100% more than normal hours
The new contract:
Basic salary receives a 10% increase
Bonuses paid depending on the number of weekends worked; the maximum would be 10% of basic salary at the end of the year if someone works one in every two weekends
Night work earns an extra 37% more than normal hours
Speaking to Premier Tom Jackson said: "Out of eight posts at my level three doctors are missing. Out of nine posts at the next level down from me four doctors are missing and at the level below them, which should be staffed by three brand new doctors that started in August, only two even started.
"We're losing people left, right and centre. I've never seen a first-year doctor post not be filled. We're at half staff and we can't go on like that."
Alex Cripps, another junior doctor and a Christian, told Premier: "Some people would try to paint it as medical people turning their back on something which God has called them to ... but I don't think that's fair because everybody's doing it in good faith.
"They either genuinely are doing what they think is right for the Health Service corporately... or they're doing what they think is right for them individually and for their family finances."
A Department of Health spokesperson said: "As doctors' representatives, the BMA should be putting patients first not playing politics in a way that will be immensely damaging for vulnerable patients.
"What's more, the BMA must be the first union in history to call for strike action against a deal they themselves negotiated and said was a good one."
Health Secretary Jeremy Hunt has called the move "devastating for the NHS" and "the worst strike in NHS history".
He has also called for the BMA to re-enter negotiations rather than striking and commended the hard work junior doctors do for the NHS.
Listen to Premier's Hannah Tooley speaking to Tom Jackson:
Listen to Premier's Aaron James speaking to Alex Cripps:
