Boiler fires up then shuts down after 2-3 seconds?

I have an Ideal Classic RS360 boiler. The system has been working since installation over three years ago and the system has not been disturbed. Now when the boiler lights up it burns for about 2 -3 seconds, shuts down and relights again so on. The boiler will light eventually but seems to go through this cycle for about a minute or two.