The veteran forecaster claims London will see temperatures of 30C on up to 50 days a year, an increase from the current rate of seven.
He says the chances of white Christmases in the UK will be reduced - with only the highest Scottish mountains experiencing snowfall in the future.
Mr Fish, who is forever linked to a "missed" hurricane warning in 1987 - although he denies responsibility - made the latest predictions as he unveiled data of computer models based on current carbon dioxide emissions, said weather systems facing the country were "scary".
The models were unveiled during a meeting of Greens at Dorchester's Corn Exchange, where Mr Fish was the star speaker for the launch of Dorset Climate Week.
Addressing an audience of 300, Mr Fish forecaster sweltering summers and drastic water shortages that would play havoc with current farming practises.
"We could well see a six degree increase in temperatures over southern Britain by the end of the century," he said, adding that water supplies would be seriously depleted.
"We could see 30C days in London increasing from the current rate of seven days a year to 50 days a year.
"And by the middle of the century, apart from on the highest Scottish mountains snowfall could be unknown in the rest of the UK. It's scary."
Mr Fish's lecture, entitled Climate Change, The Ultimate Weapon of Mass Destruction was organised by Communities Living Sustainably in Dorset.
The charity, together with Dorset County Council, revealed a five point plan to lessen carbon dioxide emissions in an attempt to slow climate change in the county.
The plan includes car sharing, cycling instead of using the car or bus, regularly reading energy meters or using an energy monitor, switching to a green energy supplier and growing food.
In the lecture, Mr Fish pointed out that 12 of the 13 hottest years in history have occurred in the last 12 years.
"The last time that the Earth was as warm as this was 120,000 years ago," he said.
According to weather statistics, Mr Fish said, the world should have been heading instead to a new Ice Age.
"And we were," he said, "But we're not now and the reason is because we have completely reversed the trend.
"We should be half a degree cooler but we've already increased it by one degree."
Mr Fish also painted a frightening scenario for a rise in sea levels that could devastate low-lying seaside communities like Weymouth, West Bay and parts of Lyme Regis.
"If we do nothing about climate change and just stay as we are now, sea levels could rise by a metre by the end of the century," he said.
However, Mr Fish acknowledged those in the audience would not known whether the computer model predictions were correct.
'"It won't be in any of our lifetimes whether we'll know if the models are right or wrong," he said.
"I just hope that there is someone somewhere, some genius, who will come up with a new energy source that we haven't thought of.
"What we need is genius."