ZURICH (REUTERS, THE TIMES, LONDON) - Global footballers' union FIFPro has reiterated its call for a mandatory rest period of at least four weeks between the final game of one season and the start of pre-season training for the next.
The union said that at least 15 World Cup players have taken part in Uefa club competitions less than four weeks after returning from Russia, leaving them without a sufficient close season break.
"Players are coming under too much pressure, caught between the needs of their clubs and their own personal well-being," it said in a statement.
"While FIFPro understands the financial implications of the Uefa Champions League and Uefa Europa League for clubs, the health of these players is being put at risk because of such a congested match schedule."
The group stage of the World Cup in Russia, which was officially part of last season, finished on June 28 while the final was played on July 15.
The first round of qualifying for this season's Champions League began on July 10, meaning that the two seasons in effect overlapped by a few days.
Iceland's Arnor Traustason played for Swedish champions Malmo in a 3-0 win away to Kosovar side Drita in a Champions League qualifier on July 10.
Serbia World Cup squad members Nemanja Radonjic and Milan Rodic played for Red Star Belgrade in a Champions League qualifier on July 11 and compatriot Vladimir Stojkovic played for Partizan in the Europa League the following day.
While clubs from smaller leagues are already involved in European qualifiers, big clubs are on lucrative tours abroad.
On Sunday, Manchester United manager Jose Mourinho said his team's World Cup participants should consider cutting their holidays short to help the depleted team in the opening stages of the Premier League season.
Several players from United's first-choice starting line-up, including Frenchman Paul Pogba, Belgian Romelu Lukaku and Englishman Jesse Lingard could miss the opening league fixture against Leicester City next week.
England defender Phil Jones and forward Marcus Rashford are expected to return early today - less than three weeks after their final World Cup game against Belgium on July 14 - to help prepare for the start of the season.
It is unclear what pressure they may have been under privately but a senior FIFPro source told The Times of London that some players were being subjected to "emotional blackmail" by their clubs.
In England, Clause 14 of the standard players' contract drawn up by the PFA in consultation with the Premier League and EFL states that all players are entitled to a break of "five weeks in aggregate over the year, of which three weeks may be consecutive".
The clause also states that "clubs should not unreasonably withhold permission" for a player wanting a break, "subject to club commitments and international commitments".