Thames Water has received multiple bids for the struggling business which has warned that it could run out of cash by next year.
Castle Water, a Scottish utility firm co-owned by Conservative Party treasurer Graham Edwards, is understood to have made an offer for Thames Water ahead of a deadline for potential investors next Thursday.
Infrastructure investor Covalis, supported by French firm Suez, had also reportedly thrown its hat in the ring, in a deal which could lead to the break-up of the debt-ridden water giant.
Thames is the UK's biggest water company and one in four people in the UK rely on the firm for their supply.