The Environment Agency has issued 10 flood warnings and 17 flood alerts in the southwest of England. The cause is as a rare alignment of the moon and sun, which exerts an unusually high gravitational pull that will boost spring tides to their highest – and lowest – levels in 20 years.

On Saturday, northwesterlies up to force 7 will exacerbate the problem in North Cornwall but it is thought that the worst of the threat will have passed by the time the forecast force 8 west-northwesterlies roll in on Tuesday.

Boat owners with swinging moorings are advised to check with the relevant authority that the mooring has enough rise, and those in marina berths are warned of potential groundings at low tide.

Referring to the present threat of flooding, the Environment Agency’s Paul Gainey said ‘Saturday is the pinch point. We think the North Cornwall coast will be one of the most affected areas and we already have crews there.’

A huge low, which is driving a storm surge ahead of it, is due to arrive on Monday night

Winds are set to build next week as a deep low pressure rumbles across the Atlantic this weekend. ‘We are tracking a storm but its path is uncertain at the moment,” said a Met Office spokeswoman, ‘but, it looks like it will bring strong, gusting winds of up to 75mph (65 knots, force 12) and gusting showers as well.’

The astronomical phenomenon, known as syzygy, could lead to tides of up to 15m (50ft) along the Bristol Channel coast and Guernsey’s Met Office has warned of tides up to 12.2m (40ft) on Saturday morning. Syzygy, which affects Earth every 18-19 years, will last throughout the year and will exert maximum effect on 21 March and 23 September, the Vernal and Autumnal equinoxes.