HENGISTBURY Head will become an island and much of Christchurch will be under water within the next 50 years if nothing is done to stem the erosion of the fragile headland.

But it could happen much sooner if a major storm breaches the Double Dykes area to let the open sea pour into Christchurch Harbour flooding low-lying areas of Wick, Stanpit, Mudeford and the town centre.

That's the scenario being presented to Christchurch councillors next week in a report from the Hengistbury Head Residents Association, which fears the 50-year prediction of coast protection experts has underestimated the rate of erosion.

The residents association fact file, to be considered by the council's community services committee on Wednesday, warns that as well as the relentless erosion "the real damage" will be inflicted by one or more severe storms.

And while recent weather patterns have been benign, predicted climate change will increase the strength and frequency of potentially damaging storms.

"Without protection these storms will wash away the cliffs adjacent to Double Dykes and will permit a sea breakthrough," says the report.

"It is a fact that if the cliffs are properly protected now by additional groynes and rock armour or some equivalent engineering solution, they will remain in place even when subjected to storms and extreme events."

But while engineers view the threat as long term and conservationists' bodies are happy to let nature take its course, government policy will not allow funding for Bournemouth council, which owns Hengistbury Head, to carry out costly protection projects.

Now the Hengistbury residents are lobbying councils and MPs on both sides of the harbour to press for closer monitoring of erosion and review the threat of a breach and need for preventive measures.