Labour claims £100m cost of pledge can be met by cutting cost of imposing competition and by keeping patients out of A&E

Ed Miliband is promising an appointment at a GP surgery within 48 hours of seeking a consultation and within 24 hours if the patient is in serious need.

The bold promise is likely to be deeply unpopular with doctors, and raises serious questions about whether it is feasible in the context of an NHS budget under sustained pressure. Labour says GPs who persistently fail to meet the new guarantee will lose some funding as a penalty.

Miliband's aides claimed the cost of the promise would be £100m a year, which could be found by cutting the costs of imposing competition on the NHS, as well as by keeping patients out of A&E.

The party claims £46m can be saved by ending the role of the regulator Monitor in enforcing competition.

The proportion of patients getting a GP appointment in 48 hours has fallen from 80% under the last Labour government to 40%. Nearly a quarter of people now cannot get an appointment in the same week.

Speaking in Manchester on Monday, Miliband said he is determined to put the NHS at the heart of the Labour's campaigning over the next year. He is following the commitment with a party election broadcast focusing on his visits to the NHS over the past few days.

The serious tone of the broadcast is markedly different from the widely criticised attack on "the incredible shrinking Nick Clegg" shown last week.

It is notable that the health service has dropped down the issues of public concerns since the NHS Act was passed, and Jeremy Hunt took over as health secretary focusing on the quality of care in hospitals. An expected NHS winter beds crisis did not materialise, partly due to better-than-expected planning.

Labour will promise patients the right to consult a doctor or a nurse at their local GP surgery on the same day, the right to get an appointment the same day if they need to be seen quickly, the right to a guaranteed appointment within 48 hours and the right to book an appointment more than 48 hours ahead with the GP of their choice. Labour claims £100m could pay for an additional 3m appointments annually.

Some studies have suggested a 5% increase in patients seeing their preferred GP could reduce emergency admissions by as many as 159,000 a year, saving £375m.

In April David Cameron made a rival offer promising that more than 7.5 million people in England would benefit from increased access to GPs, including seven-day opening and 8am–8pm appointments. The opening hours are to be extended using the £50m GP access fund.

In arguably the most contentious part of the Labour announcement, Miliband claimed at least £78m of the costs of the guarantee can be found from "unnecessary administration and legal fees because NHS services are now under threat from EU competition law".

He also promised to end the duplication provided by three NHS quangos – Monitor, the Trust Development Authority and Commissioning Support Units.

In his Manchester speech, Miliband said Cameron has broken the bond of trust with the electorate over the NHS.

He claimed: "People remember the promises David Cameron made at the last election: the airbrushed posters and the three letters he said he cared about most: NHS. He promised that people should be able to see their GP '24/7'. But a quarter of the public now say they can't get an appointment in the same week. It's a scandal that people are waiting that long. It is not how our NHS, the pride of Britain, should work.

"He has proved the oldest truth in British politics: you can't trust the Tories with the NHS."

His announcement added: "It's great people are living longer – but it means the NHS is having to cope with people in their 80s and 90s that it never had to cope with before. They don't simply have medical needs, but care needs which the NHS is not used to tackling."

In his speech he also conceded: "We know money will be tight. The last Labour government was able to deliver some very big increases in health service spending. The next Labour government won't be able to match that scale of increase. We will have to do things in a new way to make our health service better, to save money where we can – and make sure that every single penny is well spent."

In his three major reforms for the NHS he is promising an integrated service for mental health, physical health and care for the elderly, a preventive service not just a reactive service, by investing in care where people live, and finally a health service based on cooperation not competition.