Water Pouring out of Outside Pipe When Sink Tap Used All aspects of plumbing questions and answers, help, tips and information

[purdey]

Hi, I really cant work out why but when I use my bathroom sink taps water pours out of a small pipe outside (overflow?) It is only when I use the sink, not the bath or toilet and I have no idea why this has started happening and I cant find any info online anywhere can anyone help please?