As a young teacher in Leeds, Mr Willis was invigilating a science exam. To relieve the tedium he filled in a paper with joke answers and handed it in under a fictitious name with the message “I will give you pound;5, sir, if I get a good mark”.

Unfortunately a boy with a similar name had just joined the school and the furious science teacher dragged him to the head for punishment.

Mr Willis, MP for Harrogate and Knaresborough, said: “The kid - who had just arrived from Kenya and had very little English - was protesting vehemently that it wasn’t him, but the teacher wouldn’t listen. It was more than 30 years ago, but something I never, ever forgot.”

The story emerged as MPs considered the education Bill, currently going through Parliament, and debated the rights of the child to be heard.

The revelation led a Conservative Central Office spokesman to comment:

“Phil Willis brought shame and punishment on a child for something he had not done. It just shows the Liberal Democrats have always been, and always will be, people who let others take the blame for their failings.”

Mr Willis defended himself saying: “That is pathetic. I told it as a salutary tale. I only found out what had happened weeks later through staffroom gossip. It didn’t come out for some time because the science teacher was extremely embarrassed about being taken in by a hoax.”

In a previous appointment Mr Willis had the life-changing experience of being asked to demonstrate his caning skills on the headteacher at his job interview.