Nick wants to train to be a history teacher. There is only one snag, or - to be precise - two. One is his conviction for drink driving in 1994, the other is a caution for possessing cannabis in 2005.

“I do not want to embark on 12 to 18 months’ study to get Qualified Teacher Status if there is no chance of me getting a job at the end of it,” he says.

He is not the only one worried that past indiscretions could prove harmful to future prospects. On the TES web forums, a prospective teacher asks if he will have to declare a warning he received for racial harassment in 2001? Will this, he asks, prevent him from working in primaries?

The question of what to declare on application forms is easily settled. Anything that could appear on an enhanced Criminal Records Bureau check (or in the case of Scotland, by Disclosure Scotland) should be revealed, says Richard Bird, legal adviser to the Association of School and College Leaders.

Explaining why you did not mention something is much harder than explaining why it no longer matters, and if the truth does emerge you are liable to lose your job.

With criminal convictions, the impact on your employment prospects depends on the nature of the offence. A conviction relating to a pub fight when you were 21 may not mean you are considered a danger to children 10 years later, says Mr Bird.

“You should be given a chance to explain it, and employers should exercise their judgment,” Mr Bird explains.

But he says there are worrying signs that some employers are starting to operate a policy of zero-tolerance. In one recent case a judge ruled that a local authority’s insistence that prospective taxi drivers should have a clean CRB check was ill-advised but not illegal. This approach is also spreading to schools, he says.

“People are being more and more cautious where it is not really justified,” he says. “If they did an ordinary risk assessment they would judge it an acceptable risk, but where child protection is concerned they are running scared. I think that is a serious mistake and they are losing good people.”

The reality now is that even an allegation that has been recorded by police can be enough to stop you getting a teaching job.

Changes to what appears on your record following the Bichard Inquiry, set up in response to the conviction of school caretaker Ian Huntley for the murders of two Soham schoolchildren, mean that allegations can now be recorded as “police intelligence” even if no charges were brought.

Whether or not an incident appears on a criminal record can hinge on the distinction between a warning or a caution.

John Howson, director of Education Data Surveys and TES website (www.tes.co.ukcareers) careers expert, explains: “That is a fine judgment at the time but it can blight somebody’s career.

“There are undoubtedly people who are turned down (for teaching) because the criteria will include whether they have anything on their record.”

The best that applicants in this situation can hope for is the opportunity to explain to a prospective employer why CRB disclosures should not affect your chances.

At worst, you may not even get that opportunity.