On 17 May 1940 the 17,000-ton liner was packed with at least 5,000 servicemen and a few civilian women and children when it was bombed off the rocky coast of Brittany. It quickly sank with the loss of more than half those lives. It was the most harrowing setback in operation "Ariel", the largely successful attempt to evacuate over 150,000 British servicemen still in France after Dunkirk. It was also the heaviest single loss of British life at sea in the Second World War.
Resistance to Hitler's armies in France was collapsing on all fronts. When the French government asked for an armistice on 16 June, Churchill assumed that his allies had stopped fighting. In fact they had not and the British had more time than they realised.
On 16 and 17 June there were frantic evacuations of British and Canadian units from St Malo and Brest. Much equipment was left behind, unnecessarily. Evacuation from Saint-Nazaire began on 16 June and about 13,000 men with stores and transport were rescued that day. The liner Lancastria, fresh back from the evacuation of Norway, was immediately ordered to France. Such was the hurry that the captain was refused permission to discharge his surplus oil.
By midday on 17 June the Lancastria was crammed to capacity. She then waited for the other ships to be filled so that they could all sail in convoy. Fighter cover was being provided but it was intermittent and just before 2pm the nearby Oronsay was hit on the bridge by a bomb. The captain of the Lancastria was getting increasingly worried but all he could do was to get the ship's lifeboats ready, just in case, and wait. The Admiralty was too scared of submarines to allow the liner to travel alone. At 3.45 the air raid alarm sounded again and very soon afterwards the Lancastria was hit and began to sink.
There were only 2,000 lifejackets on board. Most of the lifeboats were tipped over in the panic rush to lower them from the ship's rapidly tilting sides. The improvised timber staircase from the hold broke under the rush of men trying to use it and so most of those down there never had the chance to get out. Many men lucky enough to find one of the hard cork lifejackets died when they jumped 40 feet into the water and had their necks broken by the jackets on impact.
Fourteen hundred tons of oil spilled from the ship's deep tank and spread across the surface of the sea. Calcium flares broke off the rafts and drifted on to the oil. Some of it caught fire. Men choked on the oil and were burnt in the fire. Meanwhile the air attack continued, preventing most other ships from getting anywhere near the Lancastria.
On 18 July Churchill was to deliver his "Finest Hour" speech to rally the nation as the news broke that France was giving up. He would assure the people that the country could not be invaded while there was still a navy. The last thing he needed was such dramatic evidence of the vulnerability of ships to German bombing.
It was a desperate moment. Ministry of Information reports on national morale spoke of gloom and apprehension. News of the sinking of the Lancastria was suppressed. The press complied. It was not until the American press broke the news, nearly six weeks later, that the story came out in Britain.
Churchill hyped Dunkirk and hid the disaster of the Lancastria. It made good political sense at the time, but almost 60 years later the survivors and their relatives still feel aggrieved at the way their story has been edged off the record.
Tim Clayton is co-author of `Finest Hour' (Hodder & Stoughton, pounds 20)