To think it's an outrage employer isn't financially responsible?

I had a similar situation 3 years ago, although they would often withhold all pay.
I went through ACAS as did 2 other employees. 2 of us got paid, the third it went to tribunal who ruled in his favour, formally ruled he was owed approximately £18k in salary.... except they never paid.
His only recourse was going via the small claims court, but as director was Canadian/British living in Canada there really was little that could be done, it was horrific to see and I felt so guilty getting my salary.
However, the PAYE and NI were all debts of the company not us as individuals, so whilst we received our NET pay, HMRC did not seek the additional payments from by of us, that was all classed as debts of the company and written off when the company was put into receivership.

I'm really sorry you're going through this, not all start ups are like this - just those run by morons who gamble with investment and funding and think as business owners they are owed something, which is the biggest lie ever. Too many good people get screwed and too many idiots get away with what is essentially fraud. All you can do is look for another job and get out ASAP. The second the don't pay net, run away

Lots of really uninformed comments on here

No, it is not your responsibility. You are an employee with an employment contract stating your salary. The employer has duty to withhold paye & NI from your salary and they are liable if they do not.

If they tried to claim you are "self employed" that won't reduce their obligation in this instance - the nature of your work with them is "inside IR35" and therefore they would have to apply payroll withholding as if you were employed anyway.

Nonetheless you now need to leave (clearly you can't stay working there). Notify hmrc in writing of the fraud (maybe notify police as well). If you stay knowing full well they are doing this I'd worry you would become complicit.

Thank you for the '-AI' on a search, I didn't know that.

How long have you been employed there OP?
Have you raised a grievance?
Have you notified ACAS of the issue?
You may be able to take your employers to an employment tribunal. You don't need a lawyer, you can self represent. Citizens Advice may be able to help. Contact your local branch.
More info here https://www.citizensadvice.org.uk/work/grievance-meetings/dealing-with-grievances-at-work/

https://www.citizensadvice.org.uk/

Citizens Advice can help point you in some circumstances - free and worth a go

You can put -AI in your search and it will do it without, or you can ignore the AI result and instead look for one which isn't made up. AI just makes things up and can't be trusted

If all else fails, you can, if you have to, make payments to HMRC to bring your NI payments up to whatever you should have paid in a financial year. I think you can only do this after the end of that financial year, so if you are short during this year (which ends on 5th April 2026) you can make a payment after that (by which time, hopefully, you will be working for a legit company and getting paid properly). Phone HMRC about this anytime, to get them to confirm what I've said, and talk you through the process.
Hope it doesn't come to that though.

Do you have a contract which shows your salary so you can prove you’ve been receiving the net rather than gross pay?

Citizen’s Advice Bureau

If I understand correctly you are receiving your expected net salary, proven by previous payslips? I would resign after your next pay date and sue. HMRC cannot wash their hands of this. Your company HAS to pay tax and NI on your income if you are employed, and you can prove that via previous payslips where it HAS been deducted and paid over to HMRC. Your company is a shower of shit and you need to run and go to CAB perhaps initially but I think you need a lawyer.

If you have payslips that is evidence of tax deducted by the employer. You are protected against the employer not paying it over. Without payslips you are not.

Have you spoken with Payroll/HR?

”Hi. My Government Gateway account is showing that my NI hasn’t been paid since May, and HMRC are telling me that I owe the back pay. They’re also telling me that I owe taxes on what I believed was my net pay since May—it now appears that it was my gross pay, and I’m responsible for the back taxes and back NI payments. Could you please ensure that I receive my gross pay from May-September do that I can make those payments?”

That’s your proof for hmrc get back in touch screen shot your wages on bank statements don’t let them fob you off.

This is a very strange state of affairs.
This company are obviously in real financial difficulties and has all sorts of problems.

They sound as though they are trying to syphon off as much money to line their own pockets before they go bust.

Can you and your work colleagues get it together to go and see a Criminal Lawyer who specializes in Fraud. Make sworn statements etc.

Also report them to the Police right now.

You are responsible for your own tax so unfortunately you’ll owe tax on the ‘net’ amount you’re receiving. What a shower of shits they are. If you have a contract, would a civil claim against the employer be an avenue to explore?

Good luck Op.

This is bollocks. It’s your employer’s responsibility. A very basic google tells you that.