As a cross-party group of MPs who have come together to campaign for a new settlement for the NHS and social care, we welcomed last week’s report by the Institute for Fiscal Studies (IFS) and the Health Foundation.

The next 15 years will see an extra four million people over the age of 65 in the UK. We believe that the task of meeting the health and care needs of our growing and ageing population is the greatest domestic challenge facing policymakers over the coming decades. There has been a failure to grasp the true scale of this responsibility or to plan for the number of people living with multiple long-term conditions and the escalating costs of treatment.

The IFS report, commissioned by the NHS Confederation, offers a robust and credible analysis of the pressure this will place on the NHS and care system, which is already stretched to the limit, and of the implications for public spending on these services. Its message is simple: if we want effective and safe services, we will have to find the resources to pay for them.

The report found that “a modernised NHS could require funding increases of 4% a year over the next 15 years”. That would mean health spending rising from 7.3% of national income today to 9.9% in 2033-34. For adult social care, spending is likely to have to rise by 3.9% annually over the same period.

It also offers another scenario, with modest increases in real-terms spending that will do little more than keep the show on the road – never mind deliver the higher standards of care we should expect from an institution that has long been the envy of the world.

The report presents us with a stark choice as a nation. If we want to have the benefits of a modern, efficient and effective healthcare system, which treats mental and physical health equally, tackles health inequalities and prioritises prevention, we will have to spend significantly more.

We are clear that the government should accept the case for meeting the ambitious scenario that would deliver a modernised NHS. The funding projections in the report set a benchmark against which we should judge any announcements from the government about extra funding for the NHS and social care, as we approach the 70th anniversary of the NHS.

Across the political divide, there is almost universal agreement that current funding levels are not sustainable. Few people now deny the blunt reality that, without a fresh settlement, we will face a decade of misery in which the sick, elderly and vulnerable will be repeatedly let down. Whatever your politics, that is a shameful prospect for a country with one of the world’s largest economies.

We can’t accept a second-rate healthcare service in which people with mental health problems continue to face delays and inequality, there is a postcode lottery for access to new treatments, and staff are pushed to the brink.

We need honesty and a wider public debate about what sort of services we want, how much they will cost, and how we should pay for them.

For the last two years, we have been arguing for political parties to put aside their differences and work together to answer these questions, and put our treasured NHS and social care services on a sustainable long-term footing. This may seem like blue-sky thinking given the NHS’s status as the nation’s favourite political football. But politicians, health professionals and the public are waking up to the fact that a cross-party approach is the only way we can have the mature, rational debate needed to come up with a lasting solution to this crisis.

The tide is turning. In March, more than 100 MPs from Labour, the Conservatives and the Liberal Democrats urged the prime minister to establish a cross-party commission to confront these challenges in a spirit of collaboration.

If we reject a cross-party approach and carry on as normal, political parties will continue to duck the challenges, shying away from bold solutions in favour of short-term political advantage. Abuse will continue to echo around the House of Commons. Meanwhile, adults and children across the country will continue to suffer from ever-longer waiting times, the rationing of treatment, and a growing catalogue of failures of care.

We believe in the founding principles of the NHS: a universal tax-funded service, free at the point of need, based on need, not ability to pay. We believe additional funding must be progressive and fair across the generations. People are likely to be willing to pay more if they know that these resources will be dedicated to the NHS and social care.

Last week’s report must act as a wake-up call to the Treasury and No 10. We urge the government to act now in the interests of the country – to grasp this unique opportunity to work with others and set the NHS and care system on a new trajectory for the next 70 years.