PIF members MS Society and Blood Cancer UK have teamed up with Kidney Care UK and other charities to speak out on planned changes to COVID-19 testing.
The charities are calling on the UK Government to continue free tests for people who are eligible for COVID-19 treatments, their households and personal contacts.
They say tests should be provided regardless of whether these people have symptoms.
From 1 April, the UK Government is ending free COVID-19 tests, except for people at highest risk of severe illness.
The MS Society says this means family members, friends and carers run the risk of exposing vulnerable people to infection unless they can afford to pay for a test.
There are 1.3 million people eligible for treatments like antivirals if they get COVID-19.
This is because the Government has identified them as at highest risk of severe illness.
Around 500,000 of these people are immunocompromised.
As part of the campaign to protect people at highest risk of COVID-19, the MS Society is asking people to email their MP and ask them to join the call.