"There is such a high level of interest in the 'Grexit' that we have decided to re-open the market," said William Hill spokesman Graham Sharpe.
The bookmaker is offering odds of 9-to-4 that Greece will leave the 19-nation bloc this year, implying a 31 percent chance. Odds of it staying in are 1-to-3, reflecting a 75 percent probability.
When William Hill stopped taking bets on 'Grexit' earlier this year the odds were 3-to-1, the shortest they had been up to that point. That reflected a 25 percent chance.
Explaining why it decided to start taking bets on Grexit again, Sharpe said: "People want to give us money, so we decided to re-open the book. The book-making spirit came out, and we decided to take a chance."
Rival bookmaker Paddy Power is offering odds of 2-to-1 on 'Grexit', implying a 33 percent likelihood. Ladbrokes has closed its book.
The growing threat of a default has been the talk of financial markets for the past week, and the 'Grexit' debate is in full swing on prediction markets sites that allow punters to bet against each other.