One in three GPs surgeries will take place in the industrial action (Reuters)

Three-quarters of GP practices will remain open during the doctors' strike on 21 June.

Early figures revealed by the GP magazine Pulse reveal a significant blow to the British Medical Association's (BMA) pensions campaign, as only 22 percent of practices have informed their primary care organisation (PCT) that they will take part in the strike.

In the lead-up to the first industrial action by doctors since 1975, Pulse discovered that just 281 out of 1,265 practices have taken action to show that they are taking part.

Dr Greg Place, a GP in Nottinghamshire, said doctors were concerned about the increase in their workload brought on by the industrial action, despite strong feelings on pension changes.

"We will have to do everything we don't do on Thursday on Wednesday and Friday instead. The only people who will suffer are us," he said.

A letter sent by PCTs to practices in London warned doctors that they were obligated to provide a "full service" despite any strike action.

"The local NHS may decide to withhold certain payments due to a contract holder by way of compensation for any breach, should it occur. In addition, formal contract breach notices would be issued," it warned.

In contrast to GP surgeries, four out of five hospitals and thousands of patients will be affected by the action, according to the British Medical Association (BMA).

Health secretary Andrew Lansley has warned that the strike will force up to 30,000 operations, 58,000 diagnostic tests and 200,000 outpatient appointments to be rescheduled.

Doctors insisted that industrial action would not cause sick patients to be turned away in emergencies or situations requiring surgery. However, the NHS employers organisation stated that patients were "understandably worried" about how a strike would affect their care.

The doctors' protest reflects discontent with the government's planned shake-up of their pensions. By 2014, some doctors will have as much as 14.5 percent removed from their pay for their pension, almost double the 7.35 percent taken from the pay of senior civil servants on similar salaries.

Lansley has maintained that pension reform is inescapable as people live longer. He claimed that the NHS pension scheme is and will remain one of the best available in the country.

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