Problems with business partner

[Mike42]

I'm in business with a partner (50/50). Our business also has a trade partner to which we are tied to but run financially independantly from. My partner is neither an employee or director of the other company although he does own a major shareholding.

It recently came to light during business discussions with common third party that my partner doesn't want me to see the turnover and profit figures for our partner company (information he had to hand) even though ours have been fully exposed to them. I thought we had nothing to hide between us and we should know more or less what each other's income is.

In conjunction with some other information I have since discovered, it looks like my partner has been doing work for the other company for which we haven't seen a penny. Something my partner denies. This isn't a triffling sum as it probaby runs well into 6 figures over 10 years.

This isn't just a simple case of moonlighting. Whilst telling me he's been doing non chargeable work for a customer, the other company has been secretly billing them and keeping the income!

Is there an easy answer for where I stand here? (Jokes aside!) Is there any angle where this could be considered a criminal matter? eg. fraud. Or would that only be applicable if I had a written agreement which explicitly stated he wouldn't do it?

Is my only option to take him to court if it comes to that? Would that even be a waste of time?

Sorry if that all sounds confusing. Thanks for any advice.