Israeli officials are concerned that the European Union should give no encouragement to the Palestinians to declare a state unilaterally and give no hints that Europe would support such a step.
Speaking in London after talks with UK Prime Minister Tony Blair, Mr Barak said Israel would take its own steps in response to such a declaration.
Mr Barak's visit to London and Paris comes as the Middle East peace process enters a decisive phase in the run-up to the 13 September deadline for a framework agreement on final status.
Deadline approaches
Mr Barak told reporters it was important for influential leaders like Mr Blair and Mr Chirac to be aware of the nuances of Israel's position in the peace process.
"Everything should be decided around the negotiating table, but of course I made it clear - I repeated what I've said in Israel more than once - that if unilateral steps will be taken by one side, we will have to respond with our own unilateral steps," Mr Barak said.
What that step might be, Mr Barak would not say in London.
But Israeli officials have said they will annex blocks of settlements in the West Bank if the Palestinians go ahead with a unilateral declaration of statehood.
The BBC diplomatic correspondent Barnaby Mason says such a move would probably put an end to the stumbling peace process.
Persuading Mr Chirac
Israeli officials are concerned that the European Union should give no encouragement to the Palestinians to declare a state unilaterally and give no hints that Europe would support such a step.
Mr Barak's powers of persuasion are now focussed on President Chirac, who chairs the EU and who met Yasser Arafat in Paris last Saturday.
Palestinian pressure
The Palestinians have just stepped up the pressure by formally setting 13 September for the declaration of a Palestinian state, whether agreement is reached or not.
Mr Arafat has been using the threat of declaring independence as a means of pressuring the Israelis to make concessions at the negotiating table.
Summit or no summit
Mr Barak and Mr Arafat are under intense pressure from Washington to attend a Camp David-style summit at which last minute disagreements can be hashed out face to face ahead of the September deadline.
The leaders have yet to agreed to attend such a summit, which would be chaired by President Bill Clinton.
Asked on Wednesday about the chances of the summit going ahead, Mr Barak replied: "More than 50%."