The American super star, who has his own cycling team in Radio Shack is according to his team 'suffering from acute gastroenteritis'.
Armstrong had come from Cape Town in South Africa where he was riding in a charity race and has been confirmed for the Criterium International in a fortnight and the Circuit Cycliste Sarthe Pays de la Loire in early April.
You can see the Milan-San Remo race which was won last year by Britain's Mark Cavendish on Eurosport tomorrow afternoon.
Cavendish is being tipped for success again by his HTC Colombia team. "He's the winner of the 2009 race and he's our best option for winning the race in 2010," says HTC-Columbia team manager Rolf Aldag to his team's press office.
"Although I'm sure all seven team-mates are also capable of getting good results in their own right, they will do everything they can to support him."
After not winning a stage in the recent Tirreno-Adriatico race like Cavendish did last year, Aldag says "it was a pity it didn't happen but in a way it means less pressure on him. Nobody will be looking at us to chase now if there's a breakaway of 20 riders.
"We'll keep on assessing the situation as the race unfolds. Mark's very clever and very honest and he'll let us know exactly what's going on. He won't just kill his team-mates for no reason.
"We always put a lot of emphasis on communication between the riders in HTC-Columbia and it was a big part of how we won the race last year. It'll be a big part of our game plan again this year, too.
"Milan-San Remo is a very difficult race to predict, there are so many different interests at play and it's by no means guaranteed there will be a bunch sprint. But it could happen, and the team will be working for Mark and supporting him 100 percent. There is no plan 'B'."
HTC-Columbia for Milan-San Remo:- Michael Albasini (Swi); Lars Bak (Den); Mark Cavendish (GB); Bernhard Eisel (Aut); Matt Goss (Aus); Maxime Monfort (Bel); Michael Rogers (Aus); Peter Velits (Svk).
Picture copyright of Covsupport News Service. Credit CNS/KM