The Anglian Sovereign covers Orkney and Shetland but was sent to shadow the escort of a cargo ship to Lewis after it ran aground on Monday night.
The incident came just days after the Western Isles' tug, Anglian Monarch, was withdrawn from coastguard duties.
Other UK emergency towing vessels were withdrawn last year.
In October, the UK government agreed to short extensions to the contracts for the hire of two vessels for Scotland following a campaign by local authorities and politicians.
The funding package for the Anglian Monarch ended at the weekend and at midnight for the Anglian Sovereign.
Talks are still going on between the UK government and the oil and gas industry about how future cover for the Northern Isles might be provided and funded.
Shetland Islands Council convener Sandy Cluness had described the withdrawal of the Anglian Sovereign without a deal being finalised as a "disgrace".
However, MP Alistair Carmichael has told BBC Scotland that the vessel will stay on duty to allow an agreement to be reached.
The contract for the UK's last coastguard tug has been extended after earlier plans to withdraw it from duties at midnight were abandoned.