The stern letter will remind doctors to be "true to their word" in protecting patients, and remind them of NHS commitments on waiting times, which could be breached if operations and diagnostic tests are cancelled.
The BMA has said all urgent cases will be seen, but Mr Lansley's letter will say the loss of a day's work on June 21 must not compromise patients' waiting time rights, which set out a maximum wait of 18 weeks.
The trades union has said it balloted on industrial action because the Department of Health (DoH) had refused to agree to meaningful talks. But the DoH said its officials had met with the BMA 23 times since last August.
The action will affect tens of thousands of patients and cost the NHS at least £40m.
Critics said the action will leave patients "waiting in pain" for surgery and called doctors "greedy" for planning industrial action over a pension still worth £50,000 per year in retirement.
