Routine operations and appointments have been cancelled as medics up to consultant level down stethoscopes for 48 hours from 8am today.
They will provide emergency care only, although further strikes planned for April 26 and April 27 will see a full withdrawal of labour between 8am and 5pm on both days.
Lancashire Teaching Hospitals Foundation Trust said 689 appointments and five operations had been cancelled at Preston and Chorley hospitals.
Operations director Suzanne Hargreaves said: “We have done everything we can to minimise disruption for patients, and make sure we can provide safe and effective care during industrial action.
“We have managed to maintain some planned procedures and outpatient appointments, but we have cancelled a number of operations and clinics so we can redeploy staff to look after inpatients, and make sure we could treat everyone who needs urgent care.
“We apologise to every patient whose care has been affected by the industrial action, and we’ll do everything we can to reschedule appointments as soon as possible.”
Junior doctors are objecting to a new contract which the government says will create a truly seven-day service.
They are currently paid more for working unsocial hours at night or at the weekend. But under the proposed new contracts, the Saturday day shift will be paid at a normal rate in return for a rise in basic pay.
The dispute has become increasingly fraught, with a second legal challenge lodged this week against Health Secretary Jeremy Hunt’s decision to impose the contract.