Ladbrokes said it has closed the book on evidence emerging that Mars had ever harbored living organisms.
"Following the latest news from NASA, we think it is now likely that evidence of past life on Mars will be found in the coming years," said spokesman Warren Lush.
The odds on past life on Mars were 16 to 1 when the book closed, down from 1,000 to 1 when the first bets were taken in the 1970s.
NASA scientists said this week that its Opportunity rover probe had indicated there is strong evidence that at least one part of Mars had a persistently wet environment that could possibly have been hospitable to life.
Ladbrokes is still offering 500-to-1 odds on whether there are organisms still living on Mars, according to the Bloomberg news service. The firm also takes bets on an array of non-sporting events such as the outcome of the U.S. presidential elections.
Other British bookmakers are in the space business as well: For example, the William Hill company is offering 66-to-1 odds on whether a British prime minister or American president will confirm the existence of intelligent extraterrestrial life within a year's time. It has also offered 50-to-1 odds against humans landing on Mars before the year 2031, and 10-to-1 odds against humans returning to the moon before 2016.
In the early 1960s, William Hill offered 1,000-to-1 odds against a human landing on the moon before the end of the decade — and paid out 10,000 pounds in 1969 when David Threlfall won his 10-pound bet.
Ladbrokes said it would honor past wagers on Martian life.
This report includes information from The Associated Press and MSNBC.