Charities have said thousands of people with compromised immune systems across the UK are being "shamefully let down" amid confusion over the rollout of the third dose of the COVID vaccine.
People classed as extremely vulnerable from coronavirus due to serious health conditions have been contacting charities to report they are struggling to book "lifesaving" jab appointments - raising fears it might lead to "entirely avoidable deaths".
They have highlighted problems such as not receiving an invitation from the NHS for their third jab - and reporting fewer vaccine centres being made available compared to earlier rollout programmes.
Blood Cancer UK said the "failure" might lead to "entirely avoidable deaths" and called for the NHS to "urgently announce that immunocompromised people can go direct to mass vaccination centres" for their jab.
Sky News has been told how numerous village GP services which were part of previous vaccine rollout programmes are no longer involved - with some vulnerable patients being sent many miles from where they live into busy shopping centres in major cities.
Confusion over the terms "booster jab" and "third dose" is also said to be creating extra difficulties for people trying to book appointments.
The third dose programme is different to the rollout of booster jabs to the wider population, which is being offered to everyone aged over 50.
People with a blood cancer, such as leukaemia, have been found to generally have a lesser response to the first two COVID jabs - with some patients having an antibody response of less than 10%, compared to more than 90% for the general healthy population.
Sky News has also received reports of people receiving invitations for a third jab - despite not being eligible as they have no health issues.
Gemma Peters, chief executive of Blood Cancer UK, said: "Trying to get a third dose of the vaccine if you are immunocompromised has meant navigating a bureaucracy so confusing it has felt more like something from a Franz Kafka novel than part of a 21st century healthcare system.
"This failure has caused huge anxiety for people who have been struggling to get a third dose, and sadly it may lead to entirely avoidable deaths.
"The fact that the immunocompromised now account for one in 20 COVID intensive care patients is a stark reminder of how much of the impact of the pandemic is falling on their shoulders, now that other vulnerable groups have been well-protected by the vaccines.
"This means that by failing to properly plan the roll-out of the third dose, the government and the NHS have let down the very people who have needed them the most."
The charity said from a survey of nearly 2,900 people with blood cancer, only 44% had been contacted over their third dose by 11 October - the deadline set by NHS England for when immunocompromised people should have received such invitations.
This came despite Health Secretary Sajid Javid telling the BBC at the beginning of the month that the "vast, vast majority" had been offered the third dose.
Laura Kerby, chief executive of blood cancer charity Myeloma UK, said its helpline had been "inundated with calls from concerned patients".
She told Sky News: "The delay from the NHS in sending out letters to patients already identified as extremely clinically vulnerable and needing a third dose of vaccination has resulted in people with a lower need for the next step in the vaccination programme getting their jab first.
"Tens of thousands of immunocompromised patients have been shamefully let down.
"This is further complicated by the terms 'third dose' and 'booster' being used interchangeably by GPs and clinicians leading to uncertainty about which should be given first, if any at all.
"The government must ensure that NHS systems can swiftly and accurately identify and contact patients to make sure they are given the protection they need in a timely fashion.
"Patients should not be made to wade through red tape to convince doctors they're eligible for a third vaccine."
The 119 helpline has also been criticised with vulnerable patients reporting they are unable to obtain any assistance from the service - or have been forced to "navigate" alternative routes to book a third jab.
David Collinson, 79, has had major heart surgery and told Sky News of his difficulties booking a jab.
He said: "I tried to book online several times and the same message kept appearing - 'do not try to book online, you will be sent an appointment'.
"I contacted the vaccine helpline 119, who are apparently a private agency, and was told 'sympathetically we cannot book an appointment for you'."
He added he managed to find details of the agency who gave him his previous jabs which allowed him to "circumvent" the system to book an appointment - but had made him feel a "little guilty" of the route he had needed to take.
Business Secretary Kwasi Kwarteng told Sky News the vaccine rollout had been "the most successful thing we've done" and urged those eligible to take up the offer of a third dose of the vaccine.