Boris Johnson says he fears the EU-UK trade agreement may have been breached in a dispute with France over post-Brexit fishing rights.
It comes after France said it could stop UK boats landing in its ports if the row over licences was not resolved.
The prime minister said the UK government “will do whatever is necessary to ensure UK interests”.
Earlier, the French ambassador was summoned to a meeting at the Foreign Office over the row.
Mr Johnson said: “British fishermen should be confident in going about their lawful business and they should be encouraged to continue fishing in accordance with the [EU-UK trade] agreement.
“France is one of our oldest, closest allies and friends. The ties that bind us together are far stronger than the turbulence that currently exists in the relationship,” he told journalists on Friday evening.
Mr Johnson’s comments come ahead of a “brush by” meeting with French President Emmanuel Macron at the G20 summit in Rome this weekend.
France was angered by a decision from the UK and Jersey last month to deny licences for French boats to fish in UK waters. It argued that this was a breach of the Brexit deal.
The country then warned it would block British boats from landing their catches in some French ports next week and tighten checks on UK boats and trucks if the dispute was not resolved by Tuesday.
On Friday morning, Environment Secretary George Eustice said if necessary the UK would respond in turn, saying “two can play at that game”.
The government also said it was considering launching “dispute settlement proceedings” with the EU if France goes ahead with the “unjustified measures”.
French ambassador Catherine Colonna was summoned to the Foreign Office to be told of “the disappointing and disproportionate threats made against the UK and Channel Islands”, the government added. She departed from the Whitehall building 20 minutes after arriving.
Fishing was one of the final sticking points in the post-Brexit trade agreement.
As part of the agreement, licences would be given to vessels that could show they had fished in each other’s waters for years. But there have been disputes about how much evidence is needed.
The UK says this was the case for the rejected applications which sparked the latest row.
Tensions also rose after a British trawler was seized by France and another fined during checks off Le Havre on Thursday.
French authorities say the Cornelis Gert Jan vessel did not have a licence – something the boat’s owners deny. The captain of the scallop dredger faces a court hearing in August next year.
The UK environment secretary said the trawler had been granted a licence earlier this year and the government was “trying to get to the bottom” of why it had subsequently been taken off the list given to the EU.
Andrew Brown, from the boat’s owners Macduff Shellfish, said: “We don’t know where the error of interpretation of the licence lies… but I would believe that under normal circumstances a misunderstanding like this could be sorted out with a phone call.”