A woman is seen inside a One Euro shop in central Athens, Greece July 26, 2015. [Photo/Agencies]

Osborne said the timing would be determined by the substance of a possible accord with capitals such as Berlin and Paris, which have signalled they would prefer the question resolved before they hold national elections in 2017.

"If we have a deal that we can recommend to the British people before then, then of course we can have the referendum before then," said Osborne, reaffirming the government's line.

Britain is seeking a batch of reforms ranging from a streamlining of EU red-tape to measures to safeguard Britain's position as a global financial centre and more contentious ones including cutting welfare and in-work benefits to non-British EU citizens.

With French and other officials having said that they would find it hard to accept anything that involved outright change of the EU's treaties, Cameron last month suggested that agreement to change treaties at a later date could be sufficient.

Macron would not be drawn on whether that too would be a "red line" for Paris, saying only: "The process will depend on the results of the discussions."

France is traditionally particularly sensitive to any attempt to roll back EU labour, public health and food safety legislation. It is not yet clear whether Britain will demand changes or exemptions in these areas.

Osborne was due to hold talks with Finance Minister Michel Sapin, Foreign Minister Laurent Fabius and Prime Minister Manuel Valls while in Paris. Trips to other EU capitals to make the British case are planned in coming weeks.