The news comes as BMA staff went on 24-hour strike on Wednesday in a dispute over pay.
GPs will take industrial action for the first time in 40 years on Thursday in objection to changes to the NHS pension scheme.
According to the BMA around one in five hospitals will be affected by the action on Thursday, during which doctors will cancel rountine appointments and clinics, and provide emergency services only.
The union said: ‘planning with managers indicates that at least four in every five NHS employers in secondary care across the UK have postponed some non-urgent cases.'
Figures for the number of GPs set to take action are yet to be released by the union. However a survey by GP found that just 21% of 1,660 would be taking part in the action.
The BMA has reassured patients that safety will be doctors ‘absolute priority’. The union will publish adverts in 80 regional newspapers on Thursday, explaining the action.
Ahead of Thursday’s action, on Wednesday BMA staff who are members of the GMB union will go on 24-hour strike, after the BMA refused to negotiate an improved pay offer for 2012, GMB said.
GMB represents 350 staff, including regional officers, press officers, legal, administration staff, and industrial relations officers.
Staff will hold pickets at BMA locations across the UK, after they rejected a pay offer of 1.5% plus an additional 0.5% for high performers only.
Speaking ahead of doctors’ action on Thursday BMA chairman Dr Hamish Meldrum said: ‘We are not expecting members of the public to support the action, but we hope they can understand why doctors have been driven to this point – for the first time in 40 years.
‘Patient safety is our absolute priority. We have been clear throughout that any emergency care – or other care urgently needed by patients – will be provided. We are undertaking this action with extreme reluctance.’
- Is your practice taking part in industrial action tomorrow? Email news reporter Abi Rimmer (firstname.lastname@example.org) with your stories and pictures.
- Editor's blog: Who loses if patients suffer as a result of pensions industrial action?