Campaigners have claimed that Clyde Coastguard will be closed "to all intents and purposes" from Friday evening.
After 20:00 GMT, emergency calls will be handled from Belfast or Stornoway. Routine calls will go to Belfast.
Staff at the Clyde centre, in Greenock, will be on hand until 18 December to support their colleagues.
A Maritime and Coastguard Agency (MCA) spokeswoman said the change had been planned "for some time".
A document leaked to the Coastguard SOS Campaign appears to outline details of the change.
It states: "Clyde closes on 18th December 2012.
"However, the work to complete the aerial transfers to MRCC Belfast and MRCC Stornoway is on schedule to complete on Friday of this week and the intention is to hand control of the aerials to Belfast and Stornoway on Friday evening."
Dennis O'Connor of the campaign said: "This effectively means that Clyde will cease to exist operationally from tonight.
"This move by the MCA is a further direct challenge to the Transport Select Committee who recently voiced serious concerns that the closure programme had already begun in September with the closure of Forth Coastguard despite assurances that the replacement system of operation would be fully tested before any closures took place.
"This further development shows that there is an apparent desire by some to rush through the closure plan and we urge Members of Parliament to ask the Secretary of State for Transport to investigate the tactics that are being employed by his departments."
An MCA spokeswoman confirmed the timing of the change.
She said: "This handover period has been planned for some time. All the staff have been informed well in advance."