It would be illegal for them to take this type of information into account when making hiring decisions - it would be 'direct discrimination':
It is against the law to discriminate against anyone because of:
- being or becoming a transsexual person
- being married or in a civil partnership
- being pregnant or having a child
- race including colour, nationality, ethnic or national origin
- religion, belief or lack of religion/belief
- sexual orientation
These are called ‘protected characteristics’.
You’re protected from discrimination in these situations:
- at work
- in education
- as a consumer
- when using public services
- when buying or renting property
- as a member or guest of a private club or association
As other people have already said, many employers collect this information (while keeping it secret from the people making the hiring decision) in order to ensure that their recruitment process doesn't contain systemic 'indirect discrimination', which is:
putting rules or arrangements in place that apply to everyone, but that put someone with a protected characteristic at an unfair disadvantage.
If you leave that section of the form blank then it should not count against you in your job application (often the form will say this on it somewhere).
There is an exception in that 'positive discrimination' is (since a change in the law quite recently) allowed in certain, quite limited, circumstances:
Employing people with protected characteristics
You can choose a job candidate who has a protected characteristic over one who doesn’t if they’re as suitable for the job and you think that people with that characteristic:
- are underrepresented in the workforce, profession or industry
- suffer a disadvantage connected to that characteristic (eg people from a certain ethnic group are not often given jobs in your sector)
You can only do this if you’re trying to address the under-representation or disadvantage for that particular person. You must make decisions on a case by case basis and not because of a certain policy.
You can’t choose a candidate who isn’t as suitable for the job just because they have a protected characteristic.
When recruiting you can treat a disabled person more favourably than a non-disabled person because of their disability.
See also: The Equality Act 2010 and positive action - Commons Library Standard Note
If you find that an employer is breaking the law in respect of discrimination, then contact the Equality Advisory and Support Service (free).
Some examples of the types of issues we have advised on:
- An individual who was unhappy about the way that the younger clientele at work treated him and spoke to him because he was an older person.
- A Trans individual, who had transitioned from male to female, who worked for a security company and reapplied for a security pass only to discover that the process for renewing her pass had disclosed the fact she had undergone gender reassignment surgery.