British computer experts acknowledged defeat on Friday after a German amateur radio enthusiast won a challenge to crack secret messages encoded by a World War Two cipher.

Joachim Schueth, from the German city of Bonn, managed to intercept a special radio transmission and decipher a super-complex code in less than two hours using software he wrote for the challenge.

Britain's Colossus computer, built in the 1940s to break secret German transmissions during the war and painstakingly rebuilt over the past 14 years, was still racing through its computations to come up with a solution.

Schueth's computer program actually managed to crack the hardest part of the challenge -- deciphering the code of a Lorenz SZ42 encryptor, which has approximately 16 million million million permutations -- in just 46 seconds.

It's a brilliant piece of work, really really impressive, said Andrew Clark, director of Britain's National Museum of Computing, which designed the challenge and is overseeing the running of Colossus, based at Bletchley Park outside London.

He's used a program that is highly optimized for this task and he's designed it very well.

We're really pleased and very impressed. It highlights the strength of the international community working together.

Schueth was not immediately reachable for comment, but on his Web site he explained in a very low-key way how he had gone about defeating a machine that in its day was the most powerful calculator in the world and the forerunner of modern computing.

Putting Colossus in a competition with modern computers may be a bit unfair, he wrote.

Colossus was an ingenious construction and a landmark in the history of computing. But technology has very much evolved since: When fed with a usable ciphertext, the quick-setting program ... found the setting of all 12 wheels within 46 seconds.

The Lorenz cipher is based on wheels that can have an almost infinite range of settings.

Colossus, a truck-sized computer built in 1943-4 with the help of mathematicians such as Enigma code breaker Alan Turing, has worked out five of the wheel settings so far and is expected to complete the task in about six hours.

Clark was full of praise for Schueth, who German scientists who helped design the challenge said they had never heard of. He hopes Schueth will visit Bletchley Park to receive a reward.

Colossus was a top-secret project during World War Two -- it only came to light in the 1970s -- that helped crack secret German commands on troop movements and supplies. Winston Churchill, Britain's war-time prime minister, credited it with helping to shorten the war by up to 18 months.