In the first of a series throughout the school holidays suggesting days out from Havering, Prof Ged Martin tells how Britain’s longest pier kept vital supplies moving during the war

If you've ever strolled along Southend Pier, you probably didn't realise you were walking on the deck of a British warship.

When war broke out in 1939, seaside piers around Britain were blown up as a precaution against invasion. But Southend Pier, the longest in the world, was taken over by the Royal Navy, and played an important part in the war effort.

Southend's first wooden pier was built around 1830, from the town's end out across the notorious mud. It collected trippers from holiday steamers that docked in the deep water of the Thames estuary. When it was rebuilt as an iron structure in 1889, electric trains replaced horse-drawn trams. (Since 1986, the trains have been diesels.)

The pier, codenamed HMS Leigh, organised 3,367 convoys over five and a half years. The estuary was divided into invisible squares, like a giant chess board. Each ship had a designated place. All had to set sail at the same moment and at the same speed.

Southend Pier was a gigantic filling station. Ships queued at the Pier Head for fresh water piped from the shore. Ship radios ran on batteries: Southend Pier was a gigantic recharger. Convoys were protected from dive bombers by sausage-like barrage balloons. These were inflated on the pier.

Shipping moved in both directions. The Thames was a vital supply route for London's food and fuel. Without coal, Barking and Battersea power stations couldn't have operated.

Early in the war, on November 22, 1939, the Germans launched a 90-minute air raid. Blazing away, the pier's defenders drove them off.

The Germans dropped mines that night. Some stuck in the mud, to be defused by brave bomb disposal officers - who learned the secrets of a new weapon.

The Germans never returned in force. It's one of the mysteries of the war.

By 1943, steel towers like oil rigs were planted out to sea, carrying a protective screen of anti-aircraft guns. Twenty years later, some became pirate radio stations.

Southend residents were forbidden to use binoculars, and reminded of the slogan: Careless Talk Costs Lives. In 1944, they watched the preparations for D-Day. Nobody said anything.

Gigantic concrete caissons were floated down the Thames, 135 of them. These were emergency breakwaters that formed the Mulberry Harbours, landing munitions and supplies on the Normandy beaches until the port of Cherbourg could be liberated. Nobody said anything.

One day an enormous steel cotton reel appeared on a barge. This was part of PLUTO, Pipe Lines Under The Ocean, a device like a garden hose that unrolled a pipe on the seabed so petrol could be pumped across the Channel. Nobody said anything.

On June 5, 1944, D-minus-1, there were 203 ships sitting off the pier. German spy planes flew high overhead. A handful of mines, a salvo of torpedoes would have caused chaos. Nothing happened.

At 2am on D-Day, the armada slipped away, hugging the coast past Dover, heading for the Isle of Wight rendezvous - "Piccadilly Circus" - and on to Gold and Sword Beaches.

Southend held its breath. Would they get ashore in France? Was it a Nazi trap? Three days later, the first transports returned, signalling the numbers of German prisoners they carried. The crews on the pier were exhausted but jubilant.

In eleven months after D-Day, thousands tanks were shipped from Southend. Each day, 130 ships sailed to support the Allied Armies as they liberated Europe.

Of the 84,297 ships that passed through Southend, there was only one major casualty. The Richard Montgomery, an American munitions ship, was wrecked on the Kent side of the estuary. It was too dangerous to salvage, and 1,500 tons of explosives - probably still live and maybe unstable - remain on board.

The RAF won the Battle of Britain. The Royal Navy won the Battle of the Atlantic. Southend Pier won the forgotten but vital battle to keep the Thames operating.