Insecure work in the UK is said to have reached "crisis" levels after figures showed that the number of workers on zero-hours contracts has increased by 104,000 to a record 801,000.

The Office for National Statistics (ONS) said 2.5% of the employed UK workforce were on zero-hours contracts in the quarter to last December, up from 2.3% in the same period of 2014.

The data showed there were around 1.7 million contracts that did not guarantee a minimum number of hours in November, confirming that many workers are on more than one zero-hours contract.

Unions and Labour attacked the contracts as "unjust", while zero-hours workers called them a "nightmare".

Comments posted on the jobs site Glassdoor revealed the difficulties of working on a zero-hours contract, under which workers do not know from one week to the next how many hours they will be offered.

One former worker at retail giant Sports Direct, which employs a large number on the controversial contracts, said: "Never worked longer than a 4 hour shift because they didn't want to have to give me a break."

A sales assistant in London wrote: "The 0 hour contract is bad because you can end up not getting work for days, or even weeks."

A part-time shop worker described the contracts as a "nightmare", saying staff wanted four-hour contracts to be doubled.

Unison general secretary Dave Prentis said homecare workers were among those forced to accept the insecure employment.

"These unjust contracts increase uncertainty and undermine confidence. Many homecare staff end up working when ill because they're denied sick pay. This poses a risk to people receiving home care, but workers have no choice given their shockingly low wages."

People on zero-hours contracts were more likely to be young, part-time, women, or in full-time education when compared with other people in employment, said the ONS.

On average, someone on a zero-hours contract usually worked 26 hours a week. Around one in three people on a zero-hours contract wanted more hours, with most wanting them in their current job, as opposed to a different job which offered more hours.

In comparison, 10% of other people in employment wanted more hours, said the ONS.

TUC general secretary Frances O'Grady said: "Zero-hours contracts may be a dream for cost-cutting employers, but they can be a nightmare for workers."

Jon Ingham, of Glassdoor, said: "The most common reason that unemployed people turn down zero-hours contracts is the need for a guaranteed level of income to make this a viable alternative to receiving unemployment benefit. These contracts favour the employers over the employees.

"With 38% of these contracts held by 16 to 24-year-olds, it means there is now a significant proportion of the young workforce without guaranteed incomes."

Owen Smith, shadow work and pensions secretary, said: "The scale of the crisis of insecure work under the Tories is getting worse with every passing week.

"Spiralling numbers of British workers cannot be certain where their next day's work is coming from, making it virtually impossible to plan finances and family life."

Laura Gardiner, of the Resolution Foundation, said: "It's increasingly clear that zero-hours contracts are here to stay."