Bitcoin mining is vulnerable to attacks by colluding miners, as shown in this paper. The Bitcoin protocol is not incentive-compatible, allowing colluding miners to earn more revenue than their fair share. This leads to a scenario where rational miners join the colluding group, increasing its size until it becomes a majority, thereby compromising the decentralized nature of Bitcoin. The paper introduces the "Selfish-Mine" strategy, where a mining pool keeps its blocks private and intentionally forks the blockchain. This allows the pool to gain a competitive advantage by publishing blocks when the public chain catches up. The strategy leads to higher revenues for the selfish pool, incentivizing more miners to join, which can eventually result in a majority and control of the blockchain. The analysis shows that the threshold for a selfish mining pool to gain an advantage is lower than previously thought. The paper proposes a modification to the Bitcoin protocol that raises this threshold to 1/4, making it more secure against selfish mining attacks. This change is backward-compatible and progressive, allowing partial adoption without requiring full system-wide changes. The study highlights the importance of designing protocols that prevent colluding miners from gaining an unfair advantage. The Bitcoin system is at risk if the threshold remains low, as it allows a single entity to control the blockchain. The proposed modification aims to enhance the security of the Bitcoin system by increasing the threshold for successful selfish mining attacks.Bitcoin mining is vulnerable to attacks by colluding miners, as shown in this paper. The Bitcoin protocol is not incentive-compatible, allowing colluding miners to earn more revenue than their fair share. This leads to a scenario where rational miners join the colluding group, increasing its size until it becomes a majority, thereby compromising the decentralized nature of Bitcoin. The paper introduces the "Selfish-Mine" strategy, where a mining pool keeps its blocks private and intentionally forks the blockchain. This allows the pool to gain a competitive advantage by publishing blocks when the public chain catches up. The strategy leads to higher revenues for the selfish pool, incentivizing more miners to join, which can eventually result in a majority and control of the blockchain. The analysis shows that the threshold for a selfish mining pool to gain an advantage is lower than previously thought. The paper proposes a modification to the Bitcoin protocol that raises this threshold to 1/4, making it more secure against selfish mining attacks. This change is backward-compatible and progressive, allowing partial adoption without requiring full system-wide changes. The study highlights the importance of designing protocols that prevent colluding miners from gaining an unfair advantage. The Bitcoin system is at risk if the threshold remains low, as it allows a single entity to control the blockchain. The proposed modification aims to enhance the security of the Bitcoin system by increasing the threshold for successful selfish mining attacks.