By Julien Pretot

PARIS, June 19 (Reuters) - Tour de France organisers ASO (Amaury Sport Organisation) have threatened to pull their races from the International Cycling Union calendar because of a lack of progress in the UCI's reform programme, Reuters has learned.

A source close to the UCI management committee told Reuters that ASO sent a letter to the UCI saying they would withdraw from the 2016 calendar if the reform does not go forward, with their chief concern centred on ensuring major race dates do not clash.

There is no immediate risk of the Tour and ASO's other major races not taking place but they could do so outside the umbrella of cycling's governing body, leading to a potentially damaging split.

ASO own the Tour de France but also the top tier classics Paris-Roubaix and Liege-Bastogne-Liege as well as the one-week Paris-Nice and the Criterium du Dauphine.

(Editing by: Mitch Phillips)