The move by Downing Street comes amid growing tensions between the Island and France over post-Brexit changes to rights to fish in Jersey waters.
Earlier this week, France's Minister of the Sea, Annick Girardin, threatened to cut the island's electricity supply from her country unless changes were made to the licensing regime for French fishing boats.
And dozens of French vessels are expected to stage a protest in Jersey waters on Thursday morning.
In a statement, the UK government said: 'This evening the Prime Minister spoke to the Chief Minister of Jersey, Senator John Le Fondré, and the Minister of External Affairs, Ian Gorst, about the prospect of a blockade of Saint Helier.
'The Prime Minister and Chief Minister stressed the urgent need for a de-escalation in tensions and for dialogue between Jersey and France on fishing access.
'The Prime Minister underlined his unwavering support for Jersey. He said that any blockade would be completely unjustified. As a precautionary measure the UK will be sending two Offshore Patrol Vessels to monitor the situation.'