I found the following in the excellent Wikipedia, an online encyclopedia:

Some opponents [of the constitution] argue that certain important rights, such as that of habeas corpus, are not provided for or recognised by the Constitution. The Charter of Fundamental Rights of the Union forms Part II of the Constitution, and habeas corpus is not explicity mentioned among its provisions. However, Article I-9(2) of the Constitution says: "The Union shall accede to the European Convention for the Protection of Human Rights and Fundamental Freedoms", Article 5 of which includes the following:

Everyone who is deprived of his liberty by arrest or detention shall be entitled to take proceedings by which the lawfulness of his detention shall be decided speedily by a court and his release ordered if the detention is not lawful.

Consequently, while the Constitution makes no explicit mention of habeas corpus, the Union must still uphold it because it is constitutionally bound to accede to the European Convention on Human Rights. Advocates of the Constitution often allege that in cases like this, eurosceptics seek to mislead the public by encouraging them to think that if the Constitution is adopted, habeas corpus will be abolished or might not be guaranteed in the future.