Huge parts of the world have become no-go areas for shipping unless owners are prepared to pay extra premiums to protect against war risks.
Insurers are insisting on extra payments to cover ships calling at ports thought to be particularly dangerous.
This is despite the cost of marine war insurance having risen by up to ten times since the 11 September attacks.
The Suez Canal, a crucial shipping route, now requires additional premiums as do ports across swathes of the Middle East, Africa and Asia.
Managing director of the marine unit at brokerage the Miller Insurance Group, Xavier Villers, said the number of countries attracting additional premiums had more than doubled over recent months.
Miller places about 35% of marine hull war risks on the London Market.
Villers said: "After 11 September, there was really a complete reappraisal of the way war insurance for ships is written.
"The first thing people realised was that there's a risk of terrorism at any port.
"In some sense you could say a ship is as likely to be attacked in the Pool of London as it is in Karachi, for example.
"A large part of the world now attracts an additional premium for a vessel calling there."
About 25 countries now commonly attract additional premiums. Six months ago the number would have been a single figure, Villers said.
In the absence of a market-wide tariff, such risks are negotiated on an individual basis and the value of the additional premiums can vary widely.
Market sources suggest a shipowner could be charged an extra 0.02% of the hull value to call at a port in Liberia, for example, but 0.5% of the hull value to call in Iraq, which is considered significantly riskier.
The value of a ship's hull can run into tens of millions of pounds.
Countries or areas generally acknowledged to have at least one port attracting additional premiums include: Algeria, Angola, Bahrain, Democratic Republic of the Congo, Egypt, Eritrea, Iran, Iraq, Lebanon, Liberia, Libya, Oman, Pakistan, Qatar, Saudi Arabia, Sierra Leone, Somalia, Sudan, Sri Lanka, the Suez Canal, Syria, the United Arab Emirates and Yugoslavia.
Many of the underwriters offering marine cover are also involved in the aviation market. They will have incurred large losses from the US terrorist attacks and a string of recent tragic accidents.
This is likely to exert upward pressure on rates.
The increase in the price of war risk cover and the number of countries on the list will come as particularly bad news for ships hopping between many ports on long voyages.
Villers said: "We cannot shelter owners from the increases in premium in their entirety, but we can shelter them from the worst of it.
"A good broker, with good quality clients and long term relationships in the market is able to achieve smaller increases than somebody who comes to the market new."
He predicted that in the absence of a major casualty from a war loss, rates would come down further.