Gordon Brown, the chancellor of the exchequer, said on Wednesday, the government had reasonable grounds to suspect that al-Rantisi and four others - Musa Abu Marzuq, Imad Khalil al-Alami, Usama Hamdan and Khalid Mishaal - may be involved in "terrorism".
"This action has been taken because the Treasury has reasonable grounds for suspecting that four of the individuals are, or may be, persons who facilitate or participate in the commission of acts of terrorism and one, Abd al-Aziz al-Rantisi, is or may be a person who commits, facilitates or participates in such acts," the Treasury said in a statement.
Al-Rantisi said on Wednesday that the group had no intention of attacking US interests, pulling back from retaliatory threats by the group's armed wing after Israel's assassination of the Hamas founder, Shaikh Ahmad Yasin.
Hamas dismissed the British move as a publicity stunt. "This is not the first time we hear such stories ... which only aim to deceive and mislead international public opinion," said spokesman Ismail Haniya.
Al-Rantisi said Hamas' activities were aimed solely at Israel.
The Bank of England will enforce the freeze by directing financial institutions that may hold such funds to put a stop on the accounts.