The RNLI does not believe Government assurances that Ofcom's plans for a new pricing structure for maritime radio frequencies would leave it paying less than it currently does.
In response to a written question in the House of Lords last week, Government spokesman Lord Carter of Barnes said that organisations with large numbers of transmitters would be offered a "volume discount". This, he said, would halve the RNLI's annual bill of about £40,000.
But a spokeswoman for the RNLI said the charity still feared it would have to pay up to £250,000 and would not stop campaigning on the issue until Ofcom's final proposals were published next April.
A petition on the Number 10 website calling for the RNLI to be exempted from licence fees has gathered nearly 28,000 signatures.