Parent sponsorship withdrawal when final decision has been made

[kashart]

I sponsored my parents and final decision has been made and application has been sent to London office.

My parents have not done medical yet. Now I want to withdraw my application. Can I do that at this stage or do my parents have to send them a letter that they are no more interested?

I contacted London office by email a month ago but didn't hear back from them.

Any advice will be appreciated.