UK workers on controversial zero-hours contracts experience a "deep sense of unfairness and mistrust", according to the chief of the Advisory, Conciliation and Arbitration Service (Acas).
Sir Brendan Barber, chair of Acas, said that a lot of workers on zero-hours contracts are "afraid of looking for work elsewhere, turning down hours, or questioning their employment rights" in case their work is withdrawn or reduced.
"This deep rooted 'effective exclusivity' can be very damaging to trust and to the employment relationship," Barber added.
The comments come as Acas revealed that on average it receives 70 calls a week about zero-hours contracts to its employment resolution helpline.
The non-departmental public body said that the use of exclusivity clauses did not emerge as a significant concern amongst callers, but a feeling of "effective exclusivity" did emerge as a major concern.
Acas recommend new guidance on zero-hours contracts so that both employees and employers are very clear on the working arrangements they are agreeing to in response to the government's consultation into the employment agreements in March.
The Office for National Statistics (ONS) recently revealed that there were around 1.4 million workers in the UK on zero-hours contracts in early 2014.
The employment agreements have become a focal point of political attention as the 2015 General Election draws nearer.
The Business Secretary Vince Cable has ruled out banning zero-hours contracts, but he admitted exclusivity clauses can lead to workers feeling vulnerable.
Ed Miliband promised that a future Labour government would tackle the "epidemic" of zero-hours contracts.
Miliband pledged to ban the "worse abuses of the system" by enabling workers to be free to work for other employers, have a right to compensation if shifts are cancelled at short notice and, among other things, not be obliged to be available outside contracted hours.
The government is expected to report soon on the outcome of its consultation into the employment agreements.
Case Study: Rebecca, a social care worker based in Wales
Rebecca works for a private company supporting young adults with complex care needs. She is on a zero-hour contract and her hours vary enormously. For example, one week she worked 28.45 hours, and then the next she clocked-up 56 hours. Rebecca says the amount of hours she will work is a surprise every week.
But the families of the people Rebecca and her colleagues support do not understand why they are on a zero-hour contracts. This is because the clients' care needs will not decrease – they will only ever increase.
With constantly varying hours, Rebecca explains she cannot budget. She and her family are in social housing, she can claim housing and council tax benefits. But Rebecca would prefer regular hours so she can arrange appropriate childcare.
At the moment her partner is unable to work as he cannot commit to set hours. Rebecca wants the government to make companies offer a minimum of 16 hours per week for employees on zero-hour contracts unless workers ask for less in writing.