The number of people waiting for heart surgery in England could rise by more than 40% by next spring, a charity has warned.
The British Heart Foundation said more government money was needed - or it will take up to five years for cardiac services to return to pre-Covid levels.
"Even before the pandemic began, waiting lists for vital heart care were far too long," said the BHF.
The government said it had invested £1bn this year to reduce waiting lists.
The charity's warning about heart care comes just a day after new analysis suggested nearly 14 million people could be on NHS waiting lists in England by next autumn.
A backlog is predicted as patients who did not come forward during the pandemic now seek treatment and join waiting lists for routine operations.
The BHF said the number waiting for heart surgery - such as a heart bypass or transplant - in England is predicted to rise from 11,000 to more than 15,000 by February.
The number waiting for more general heart care - for example diagnostic scans and treatment such as stents - could reach 500,000 by early 2024, it said. This would be double the levels seen before the pandemic in February 2020.
The BHF said it had carried out modelling for its worst case scenario - and estimated that cardiology waiting lists might not return to early 2020 levels until the end of 2026.
Around 7.6 million people live with heart and circulatory diseases in the UK, the BHF said.
"Delay in diagnosis and treatment of cardiovascular diseases is not just about improving symptoms, however important that is - it is about saving lives," Prof Sir Nilesh Samani, medical director at the BHF, said.
"Tragically, we have already seen thousands of extra deaths from heart and circulatory diseases during the pandemic, and delays to care have likely contributed to this terrible toll."
The charity said it wanted to see an above-inflation rise in health spending and a plan to recruit more heart specialists.
"At this critical moment, the government must act now to avoid more lives lost to treatable heart conditions. Addressing the growing heart care backlog is only the start," said Prof Samani.
"We must also see a clear plan, alongside significant and ongoing investment, to build capacity back into the NHS and address the pandemic's impact on health workers. Getting this right would avoid preventable heartbreak for many families."
The government pointed to data from May this year that showed services were already recovering - and the number of new referrals for cardiology and cardiothoracic surgery had returned to similar levels seen prior to the pandemic.
"The NHS has faced huge challenges over the past year due to Covid-19 and we continue to support our incredible health and care staff who have kept services open for thousands of patients," a Department of Health and Social Care spokesperson said.
"This includes dedicated investment of £1bn this year to tackle the backlog and reduce waiting lists."
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