When I wrote my recent piece speculating whether Bond was an influence on The Avengers, I didn't envision that post becoming a series, but this is probably the second in an unintentional series of posts on The Avengers in its context of Cold War spy literature. Perhaps I should also say that probably none of what I am going to say is academically proveable, so if you're a media student please don't quote me, I am not respectable in any way.
Oh, one last disclaimer, Michael Caine normally irritates me witless. In fact Ipcress is the only one of his films I have managed to see all the way through, and I notice a few other people say that they like the film despite its cast. You may also say that his Palmer character is almost diametrically opposed to Steed - definitely working class and not with the grand origins Steed refers to now and again.
My theory would be that The Ipcress File may have been initially influenced by The Avengers , since it came out halfway through the show's original run. Ipcress was phenomenally successful, and so I would theorise that later series of The Avengers then parodied the Ipcress File. It's all getting a bit postmodern and reflexive here, especially as Ipcress was conceived as being a sort of anti-Bond, and deliberately made to be downbeat.
An example would be that the earlier series of The Avengers actually make quite a lot of the characters' sophisticated taste in food. Take the episode where Steed is going away and gives all sorts of exquisite things he won't be eating, to Cathy. In the manner of the time, both Steed and Cathy are depicted as quite sophisticated in their tastes, which even into the sixties was a major symbol of modernity and luxury. In The Ipcress File this focuses into Palmer's gourmand taste, shown by the opening shots around his flat which focuses in on his kitchen. In the book Palmer has other luxurious tastes apparently (I haven't read it) but in the film this becomes his taste in food alone, for which he is renowned. Then The Avengers parody this even further in the later series by the introduction of endless champagne, far more than an ordinary spy would drink in the course of a day's work. Palmer is quite similar to Steed in his attitude to authority, and I would suggest that the last series of The Avengers sends up the subject of authority by making Mother the authority figure.
The pretend servant agency which serves as a cover in The Ipcress File is so Avengers I'm surprised they didn't think of it. Steed had to make do with a pretend butchers of course. There are other symbols of the gentleman of the time in The Iprcress File, for example the magnificent building used as a set for the ministry, and the way Palmer's boss looks like Steed by his outfit of suit, bowler hat and umbrella, the uniform of the gentleman civil servant or whatever at the time. Another visual similarity is the scene where Palmer is being interrogated, which is very reminiscent of a similar scene with Steed (its in one of the extant expisodes, but I can't remember which one, perhaps someone will comment).
Perhaps the likeness that I'm getting at is the atmosphere of The Avengers. Not least I think is the similarity of personality between Palmer and Steed, at least in terms of humour. The scene where Palmer is taken into a cover fireworks factory and pushes down the plunger on one of those things used to set off dynamite, is exactly the way Steed would have behaved in the setting and the sombre circumstances. I also have a feeling that The Ipcress File isprobably what The Avengers would have looked like if it hadn't been confined to studios mainly (in the early days): the London of the 1960s. Of course as we all know it came to rest in an England which was about as unreal as it could be, but that was after Ipcress. One of the reasons therefore that they feel like each other, is that Ipcress is set in the real London which was the setting of the Avengers, and which it came to parody.
Does my little theory here stand up to much scrutiny? Probably not, and of course the similarities may just be the result of the craze for all things espionage at the time. But it would be nice to think it happened like this.