Vaccine walk-in centres are defying official guidance by handing out second doses after just 21 days.
Young people in particular are bypassing the booking system and relying on word of mouth via social media to identify centres willing to bend the rules.
On Sunday, NHS officials were forced to reprimand a walk-in centre at London’s Science Museum after it emerged they were issuing walk-in second doses to people of any age, as long as it was 21 days since their first.
The NHS is currently trying to speed up delivery of first vaccine doses to combat the spread of the more infectious and dangerous delta variant, opening up jabs to people of all ages without an appointment.
However, second jabs should not be available at walk-in centres until at least eight weeks.
Social media sites such as Reddit are being used to communicate news of vaccine sites offering quicker second jabs.
One post stated: "Monday 28 June, just got second Pfizer jab 25 days after first at Emirates stadium."
Another said: "Just been to Wembley Park vaccination centre. No queue, no wait, got my 2nd Pfizer dose at 28 days. They were happy with anything over 22 days."
A source at NHS England said local centres with plenty of stock were taking matters into their own hands but that there was little the organisation could do to stop them other than asking them to follow the rules.
An NHS spokesman said operational guidance is clear that second doses should be offered from eight weeks to ensure maximum protection, which is in line with advice from the Joint Committee on Vaccination and Immunisation and the Government for people aged 40 and over.
"The NHS is also proactively contacting those who are eligible to bring forward their jab," added the spokesman.