A letter from Tang to the alliance said an initial recommendation that the group be struck off had come from police chief Raymond Siu.
Tang said that, having considered the recommendation and information from Siu, he was of the view that were the alliance a society in respect of which the Societies Ordinance applied, then it would be necessary to prohibit its operations in the interest of national security, public safety and order.
He said he would therefore pass this recommendation on to the Chief Executive.
But the secretary added that the alliance will first be given until 5pm on September 24 to make a written representation as to why it should be allowed to remain on the register.
The group, along with three of its leaders, has been charged with inciting subversion of state power.
The alliance said it will seek legal advice before responding to Tang’s letter.