In a survey of over 4,000 members of the British public, Which? Found 26% of workers only skim read their employment contracts, while 6% admitted to not having read them at all.
Only three in 10 employees received their contract before starting their job, and 9% did not get a contract until they'd been in the post for six months or more.
Overall, at least two million workers in Britain do not have an employment contract.
Which? chief executive Peter Vicary-Smith said: "Our research shows many people fail to take the time to read their employment contracts properly, which means they have no idea what they've signed up to and could be in for a shock in the future.
"[Staff should] read the contract before signing it and check that the terms - such as salary, holiday entitlement, notice period and redundancy procedure - are in line with what your employer agreed at your interview. Dotting the ‘i's and crossing the ‘t's could pay dividends in the long-term."
Mark Hammerton, partner at international law firm Eversheds comments: "The survey results disclosed today are not altogether unsurprising. Many employees will rely upon what they are told at interview or in an offer letter regarding the terms of their engagement and will not delve in the "small print" contained in a written contract.
"Such seeming employee apathy should not, however, encourage more employers to avoid providing written terms or giving careful consideration to what these should be at the start of employment. There are real risks in doing so since, in the absence of a written contract, were any dispute to arise later on in the employment relationship, it will be for the courts and tribunals to try to piece together what the employment terms were; a task frequently fraught with difficulty, time commitment and evidential burden. Employers will also be missing an important opportunity to clarify what is and is not a contractual obligation on their part. A case going through the courts currently, for example, suggests that breach of a contractual disciplinary procedure could potentially lead to significant compensation running to £100,000s in some instances.
"As a general rule, employers would want to make clear in the employment contract that disciplinary and similar procedures fall outside of their contractual obligations.
"In addition employees may also be surprised when or indeed if they do read the detail of their contracts. Many contracts will specifically state that the terms override any preceding discussion or offers, making it very difficult indeed for them to rely on what they thought were earlier promises after the event."