A pay rise would make me financially worse off, why?

[Amy0408]

Hi just wondering if anybody else has had this issue.

I'm currently earning £17,000 before tax and NI, and I'm in reciept of benefits as I'm a single parent and my child is in full time childcare. I've recently interviewed for a job which would bump my salary to £22,000. I've been onto a few. GOV benefit calculators and it shows that I would be around £3,000 better off staying at £17,000 than taking the new job with a higher salary.

Does anybody know why this is? I know that taking a higher salary would lower my benefit entitlement but surely it shouldn't leave me worse of than I am now. I feel as though the system doesn't allow people to take that next step up the career ladder.

Or alternatively, what a farsighted system that encourages people to continue to save for their retirement through the dip in earnings round young children, thereby saving the public purse from having to support them as much in old age .
We subsidise high earners' pensions by a heck of a lot more through rebating them by 40%.