The value of the care provided by carers has been estimated at £23bn
Carers have to jump through unnecessary hoops to get benefits, MPs say.
The Department for Work and Pensions pays around £2bn a year in benefits to some 900,000 people who look after relatives and other loved ones.
But around one-fifth of those carers who receive benefits say they experience difficulties claiming, the Public Accounts Committee found.
Meanwhile, a survey has found millions of people do not realise carers have the right to ask for flexible working.
The value of the care provided by people looking after friends and family has been estimated at £23bn.
But the Public Accounts Committee found when those carers apply for benefits they face a complicated and confusing system.
For example, carers who get a state pension are not eligible for Carers Allowance, but still have to apply for it, if they want other top-up benefits. The report says they should be able to apply for those benefits directly.
It also says part-time work would be the best option for many carers, but job centres are not geared to advise them on this.
The report noted that only £25m of £38m set aside last year for employment support for carers has so far been committed.
It warned the other £13m should not be diverted elsewhere in the JobCentre network as demand for help rises due to the recession.
The cross-party committee chairman Edward Leigh said: "The value of the service that these unpaid carers provide to society is not reflected in the quality of the DWP's arrangements for providing them with financial and other support.
"Carers who apply for benefits should not have to wade through official written guidance and communications which can range from the hard-to-understand to the downright incomprehensible.
"And they should not have to be jumping through unnecessary hoops to apply for benefits and allowances."
The separate survey on flexible working for the government found 82% of those quizzed were not aware that carers were legally entitled to ask for flexible working arrangements.
More than a third of those asked - 35% - had caring responsibilities which made them eligible to ask.
The research showed more than a third of women and just under a third of men are currently carers, which if repeated across the country could mean that about 9m women and 7m men are carers.
Harriet Harman, minister for women and equality, said: "We do not want people to have to choose between their work or providing important family care, so the question of their ability to work and employers' ability to respond flexibly is a huge one for the future.
"This government recognises the importance of this family care, and those who go out to work as well as care for older relatives. That is why we brought in the right to request flexible working for those who care for older and disabled relatives."
The research was conducted by ICM, in an online survey of 2,036 adults in July.
In response to the MPs' report, the Department for Work and Pensions said it would be looking at ways to simplify the system and that the importance of carers is recognised.