Chris Froome offered €1m Grand Tour challenge by Oleg Tinkov
Team Sky's Chris Froome has been challenged to compete in all three of 2015's Grand Tour races for a €1m (£789,329) prize-fund by rival team owner Oleg Tinkov.
Tinkov also made the offer to Giro d'Italia winner Nairo Quintana and Tour de France champion Vincenzo Nibali.
The Russian millionaire wants the trio to take on Spaniard Alberto Contador, who leads his Tinkov-Saxo team.
The Vuelta a Espana is the year's third Grand Tour after the Giro and Tour.
|Grand Treble contenders|
|Name||Team||Grand Tour classification wins|
|Alberto Contador||Saxo-Tinkoff||TdF: 2009, 2007, Gd'I: 2008 VaE: 2014, 2012, 2008|
|Chris Froome||Team Sky||TdF: 2013|
|Vincenzo Nibali||Astana||TdF: 2014, Gd'I: 2013 VaE: 2010|
|Nairo Quintana||Movistar||Gd'I: 2014|
The prize fund could be divided between the riders taking on all three races, or go to the man who does best across all three of the three-week races.
The route for the 2015 Giro d'Italia, revealed on Monday, raised the possibility of the quartet competing against each other before launching rival general classifications campaigns in the Tour de France a little over a month later. The Vuelta is raced in August and September.
The 2015 Tour de France route will be unveiled on Wednesday, 22 October with the Vuelta route to be published next year.
Team Sky principal Sir Dave Brailsford, whom Tinkov targeted with a Twitter message reiterating the challenge, said a series of high-profile clashes would fuel interest in the sport, even if it may prove difficult for the riders involved.
"Seeing the best riders slug it out across the season at key races is what everybody would want to see," he said.
"From a performance point of view, it's quite a serious undertaking but, from a conceptual point of view, from a fan's point of view, from a sporting point of view, I think it's got a lot of merit."
Nibali and Contador are two of six men to have won all three Grand Tours during their career but no rider has won all three in one season.
Nine men have won either the Giro and Tour, Tour and Vuelta, or Giro and Vuelta in the same year.
Contador was the last to win two races in one year when he won the Giro and Vuelta in 2008, while Italian Marco Pantani won the Giro and Tour in 1998 and France's Bernard Hinault did the Tour and Vuelta double in 1978.