In the world of gambling, Russia's presidential election is turning into a sucker's bet.

Although public favor has soured against frontrunner Vladimir Putin and the political landscape feels more open than it has in years, British bookmakers say the odds that anyone will defeat him at the polls rest somewhere between slim and none.

As opposition leaders prepare for a third major rally against election fraud this Saturday, the odds of a Putin victory increased to 1-to-20 on Tuesday from 1-to-10 the day before — meaning a 100 ruble bet will now net only a measly 105 rubles in return.

England's top bookmaker, Ladbrokes, ranks billionaire businessman Mikhail Prokhorov as the nearest contender with odds of victory at 10-to-1.

Communist Party chief Gennady Zyuganov, A Just Russia head Sergei Mironov and the Liberal Democratic Party candidate Vladimir Zhirinovsky all come in at 25-to-1 odds.

Despite the unattractiveness of the field, Ladbrokes said it was still seeing a fair bit of action from long-shot bettors.

"This has probably been more popular than we expected and has seen several thousand pounds staked on it so far," Ladbrokes spokesman Richard Royal said via e-mail.

While other English bookmakers known for taking bets on political affairs have not offered odds this year, Royal said Ladbrokes felt there was enough of an unknown element to make things "interesting."

"We did note the very public rallies against Putin, which made us consider that this would be an interesting market and that it wouldn't be as straight forward as initially expected," he said.

As for the odds against those challenging Putin, Royal was diplomatic.

"We often have a range of candidates and options featured among a market, even those unlikely to win, some are merely for fun, and usually their odds reflect this," he said.

In 2008, online gambling operator Unibet stopped taking wagers days before the presidential election because Dmitry Medvedev had outpaced his challengers by such a wide margin.

Medvedev ended up winning 71 percent of the vote that year. Putin has recently been polling at under 50 percent, still giving him a considerable lead over the rest of the field but not enough to avoid a second-round runoff.