Mac has stopped backing up to time machine

I have an external disk which i always use to back up but it has stopped working

When the backup tries to start I get the message

This back is too large for the backup disk - the backup requires 77.90 GB but there is only 8.62 GB of space.

This drive IS my standard backup disk so I don't want to remove anything as it will have an impact on my previous back ups - and yet oddly the mac is no longer seeing those

How do I regain access to the backups on this drive?