In Britain, we fast more than seventeen hours a day. Is it permissible to fast for twelve hours only?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Allaah The Almighty Said (what means): {And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset.} [Quran 2:187]

So, the fast time is clearly defined and imposed by Allaah The Almighty in a way that makes it prohibited to be lengthened or shortened.

Allaah The Almighty legislated the fast for very important rationales including refining His slaves’ souls and hearts and purifying them‎ from evil. Allaah The Almighty Says (what means): {O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous…} [Quran 2:183]

Thus, Allaah The Almighty did not ordain the fast to torture or trouble His slaves.

Before being imposed in the manner that it is, the fasting would start not at the crack of dawn but after the 'Ishaa' prayer or if one would sleep after sunset. However, Allaah The Almighty had mercy upon this Ummah (nation), facilitated the fast and allowed us to eat and drink all night. What could be easier for you than what Allaah The Almighty has made easy?

Therefore, what you have suggested in your question is absolutely prohibited because it is tantamount to a manipulation of the religious assignments and a transgression of the limits set by Allaah The Almighty who says (what means): {And those are the limits [set by] Allaah. And whoever transgresses the limits of Allaah has certainly wronged himself.} [Quran 65:1]

Moreover, reward is proportionate to the difficulty of the act of worship; the Prophet, sallallaahu ‘alayhi wa sallam, said: "Whoever fasts Ramadan out of faith while seeking its reward from Allaah The Almighty, will have his past sins forgiven." [Al-Bukhari and Muslim]

Indeed, seeking the reward of Allaah The Almighty means that you do not think of the fast as something heavy and difficult and you do not feel annoyed about it.

However, if a fasting person finds that it is unbearably difficult for him to complete his fast to the extent that he may die or something of the like, then it would be permissible for him to break his fast and then refrain from eating again until the end of that fasting day, However, the person would be required to make up that day.

We ask Allaah The Almighty to guide all of us to the right path.

Allaah Knows best.