Around 3.7 million calls to NHS 111 were dropped last year amid long waits to get through, according to new analysis.
Almost one in five callers – or 18% – gave up before speaking to anyone, with the average time taken to answer a call stretching to 25 minutes in December, research commissioned by the Liberal Democrats shows.
The party’s leader Sir Ed Davey said it is “completely unacceptable” that people needing medical advice struggled to get through to the helpline.
It comes as junior doctors across England on Tuesday launch a four-day strike in a worsening dispute over pay which threatens huge disruption to the NHS.
Sir Ed said: “It is completely unacceptable that so many people in need of urgent medical advice are struggling to get through to NHS 111. Staff are exhausted, patients are left in pain, but still Conservative ministers are burying their heads in the sand.
“The Government must urgently hire and train more staff to take 111 calls, or else millions more people will be left in pain for far too long.
“Local health services across the country are at breaking point after years of neglect and underfunding from this Conservative Government. From ambulance waiting times and a lack of GP appointments, our health services are buckling under pressure.
“The Conservative Government’s record on health has been a shambles and today’s figures are yet further proof that we cannot trust them to run the NHS.”
A Department of Health and Social Care spokesperson said: “This analysis is based on statistics from last year – since then we have published our Urgent and Emergency Care Plan to help the NHS deliver for patients which includes increasing the number of NHS 111 call handlers to 4,800.
“Our plan will deliver one of the fastest and longest sustained improvements in emergency waiting times in the NHS’s history, with £14.1 billion made available for health and social care over the next two years on top of record funding.”
