PARIS (Reuters) - Negotiations over scrapping two French national holidays will need to be concluded by September 30 at the latest, Prime Minister Francois Bayrou has said, according to a report by newspaper Les Echos, as the government seeks ways to narrow its repeated budget deficits.

Bayrou, a long-time debt hawk whose minority administration is walking a political tightrope, made the headline-grabbing proposal in July, when he outlined a series of deficit-reduction measures worth 43.8 billion euros ($51 billion) next year aimed at lowering France's debt.

The prime minister has asked for bodies including unions, government officials and others to let him know if they want to hold open negotiations by Sept. 1, according to an internal document cited by Les Echos but not verified by Reuters.

Bayrou said as part of a recent budget proposal that he wants to scrap both the Easter Monday and Victory in Europe Day holidays, leading to an outcry from parties on both the left and far-right. 

Les Echos said the letter also states that the choice of holidays identified was another issue that could be discussed.

Major trade unions CFDT, CGT, FO, CFE-CGC and CFTC signed a joint declaration on Saturday denouncing Bayrou's budget proposal and said they will hold a meeting among themselves on September 1 to decide how to organize themselves against the move.

The prime minister's office did not immediately respond to a request for comment.

Bayrou has previously compared the month of May to Gruyere cheese - full of holes - and has said reducing the number of holidays will bring economic benefits, although recent experience elsewhere and various economic studies suggest it may not be as simple as that.

($1 = 0.8592 euros)

(Reporting by Forrest Crellin and Claude Chendjou; Editing by Hugh Lawson)