Two offshore patrol vessels will "monitor the situation" after French maritime minister Annick Girardin warned on Tuesday that the country was ready to take "retaliatory measures.”
This comes after Mr Girardin accused the Channel Island of delaying issuing new licences to French boats.
Prime Minister Boris Johnson spoke to Chief Minister of Jersey Senator John Le Fondre, and the Minister of External Affairs, Ian Gorst, on Wednesday and "underlined his unwavering support" for the island.
A Downing Street spokesman said: "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.
"They agreed the UK and Jersey Governments would continue to work closely on this issue."
The UK and Jersey have already criticised France for making "disproportionate" threats after Paris warned it could cut off electricity to the island.
95% of Jersey’s electricity comes from France through three undersea cables.
The row has come after the island implemented new requirements under the terms of the UK-EU trade deal for boats to submit evidence of their past fishing activities.
This is in order for them to receive a licence to carry on operating in Jersey waters.
A UK Government spokesman earlier said: "To threaten Jersey like this is clearly unacceptable and disproportionate.
"We are working closely with the EU and Jersey on fisheries access provisions following the end of the transition period so trust the French will use the mechanisms of our new treaty to solve problems."
Mr Gorst told the BBC Radio 4 Today programme: "This is not the first threat that the French have made to either Jersey or the United Kingdom since we are into this new deal.
"It would seem disproportionate to cut off electricity for the sake of needing to provide extra details so that we can refine the licences."