A human rights group has warned insurance companies that they could be aiding terrorism if they insure ships that break the blockade of Gaza.

Israeli organisation Shurut Hadin has written to almost all major insurance companies worldwide, including Lloyd's of London, the biggest in the world.

It warns them that they could be liable for massive damages if the ships they insure break Israel's blockade around Hamas-controlled Gaza.

Lawyer Nitsana Darshan-Leitner, the founder of the group, explained: "We sent these letters to the largest insurance companies in the world, including Turkish companies, which represent over 99 per cent of the maritime insurance business worldwide. We warned them that, if they insure these ships, they could be sued by victims of Hamas attacks."

The human rights group represents victims of terrorists in courts around the world. Ms Darshan-Leitner said: "It has been legally established by courts in the United States and Europe that there is no difference between the military and social frameworks of terror organisations. Any supplies sent to Gaza will probably fall into the hands of Hamas. Therefore, anyone helping the flotilla is abetting terror."

She said she believed that all the previous boats which had attempted to break the naval blockade had been insured, including the three boats in last May's Gaza Freedom Flotilla.

The Turkish Islamist movement IHH, which organised last year's flotilla, is planning another, larger flotilla next month, to deliver supplies to the Gaza Strip. In the violent clash at sea last year, nine Turkish activists were killed and 40 others wounded. Twelve Israeli naval commandos were also injured.

Lloyd's said it would refuse to insure a ship if the owners were attempting to help Hamas and said that if the local law was broken by a ship insured by them, the insurance would be invalid.

A Lloyd's representative said: "Hamas is subject to UK and EU terrorist financing sanctions. As such, any vessel identified as being owned or controlled by that organisation would not be permitted to be insured by Lloyd's, or any other EU insurer. Lloyd's has robust systems in place to ensure international sanctions are followed."

The firm said it had no knowledge of ever having insured a boat attempting to break the Gaza blockade.

Israeli security sources have kept silent over recent weeks over how they plan to block another flotilla, although senior navy sources have said that the lessons from last year's incident had been learned and they were ready to enforce the closure of the Gaza Strip.

A Malaysian boat attempted this week to reach Gaza, but after Israeli ships fired warning shots, it docked at an Egyptian port. Diplomatic efforts to convince the Turkish government to stop the flotilla have so far failed.