March 1943

Colossus computer

Colossus Mark 1 prototype is completed and tested at Dollis Hill

Tommy Flowers’s diary places Colossus still in his London laboratory in mid-January 1944 — which unpicks the tidy story that it ran at Bletchley Park on 8 December.

A postal research laboratory in December 1943

During December 1943, Tommy Flowers and his General Post Office team were completing and testing Colossus Mark 1 at the Post Office Research Station at Dollis Hill in London. The date filed against this entry, March 1943, does not fit either the description or the evidence.

The prototype demonstrated that a programmable electronic digital machine using about 1,500 thermionic valves could execute Bill Tutte's statistical codebreaking procedures. It had been designed and built over eleven months, after the two-tape Heath Robinson machines proved limited by synchronisation and reliability problems.

Tested, but not yet working

The successful functional test cleared the machine for delivery to Bletchley Park, where Max Newman's section was waiting for machinery to apply statistical attacks to Lorenz traffic. Colossus was then dismantled for transport.

Contemporary records place its arrival at Bletchley Park on 18 January 1944, and its first attack on an operational Lorenz message on 5 February 1944. In December 1943 it was therefore tested but not yet doing operational cryptanalysis.

What the 8 December date rests on

December 1943 is well supported, but the exact first-run date is not. The often-repeated 8 December is presented by The National Museum of Computing as a recollection or supposition rather than a documented date.

Flowers's surviving diary shows Colossus still at Dollis Hill on 16 January 1944, and a contemporary memorandum by Newman records its arrival at Bletchley Park two days later. After the machine completed its first real job on 5 February, Flowers recorded a more ordinary disaster in the same diary: his car broke down on the way home.

Sources

Researched 23 Aug 2026 5 sources 2 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at 1943-12.

What the sources leave uncertain

December 1943 is well supported, but the exact first-run date is not securely established.
The often-repeated 8 December date is presented by The National Museum of Computing as a recollection or supposition. Flowers's surviving diary shows that Colossus was still at Dollis Hill on 16 January 1944, while a contemporary Newman memorandum records its arrival at Bletchley Park on 18 January.
The supplied date 1943-03 is incompatible with both the listed description and the historical evidence; the event belongs in December 1943.