The causeway is passable for only two hours either side of low tide

As the tide began to rise, the four went into the water next to a sign warning people not to attempt the crossing.

Last month an inquest into an outdoor pursuits leader who drowned there heard that new signs had been put up.

Five others who were trapped had used a new bell to alert emergency services.

The rescue was launched just after 1400 BST on Saturday by Swansea Coastguard off Worms Head, a tidal island reached by a causeway which is passable for only two hours either side of low tide.

A mobile phone call had alerted them to the fact that as well as those people trying to the swim, there were other people still on the island and cut off by the rising tide.

Don't play chicken with the sea, it will always win Eric Birkett, Swansea Coastguard

When the coastguard rescue teams arrived, they found the four trying to swim the causeway were in difficulty - although they did finally make it.

The other five people cut off on the island were transferred back to safety by the inshore lifeboat.

Eric Birkett, watch manager at Swansea Coastguard, said: "Bizarrely, the four that went into the water entered the rising tide nearby to very large signs which warn the public not to attempt the crossing.

"This has been the site of a recent fatality, and there is also a large bell in situ which can be rung if people are seen to be in difficulty in the causeway.

"We believe this was the first time it has been used to gain attention.

"We are therefore reminding the public to take care at the crossing point and also to remind them that the island can be reached only at low tide and only for about two hours.

"Don't play chicken with the sea, it will always win," added Mr Birkett.

The extra warning signs and the bell for those stranded to summon help have been erected following the death of Ian Reynolds, 29, of Pontarddulais in May 2005.

Mr Reynolds had been camping on the island and when he woke to find the tide rising he attempted to swim to shore, but never made it.