iv had a crypto broker for around 4 months now, I invested £9000 and my account has reached over £30,000

Hi, iv had a crypto broker for around 4 months now, I invested £9000 and my account has reached over £30,000 now, iv asked for a £4000 withdrawal and then received an a mail saying I have to pay nearly £6,000 tax before all funds are released to me, my broker said this isn’t unusual and to pay it ASAP so I can then send the majority of it back to carry on trading. This really doesn’t feel rite. Has anyone heard of this procedure before? Many thanks Carl