BBC director general Mark Thompson has confirmed plans for an iTunes-style download service that will allow viewers to buy programmes minutes after they have finished on TV.

Thompson said the proposal, called Project Barcelona, would allow viewers to "purchase a digital copy of a programme to own and keep [for] a relatively modest charge".

Thompson was not specific about the timescale or pricing, but sources said it was hoped that programmes would be available to buy at the same time as they go on the iPlayer. Early speculation put the price at £1.89 a show.

Anticipating criticism that viewers were being made to pay twice for the same content, Thompson said: "This is not a second licence-fee by stealth or any reduction in the current public service offering from the BBC – it's the exact analogy of going into a high-street shop to buy a DVD or, before that, a VHS cassette.

"For decades the British public has understood the distinction between watching Dad's Army on BBC1 and then going out to buy a permanent copy of it. Barcelona is the digital equivalent of doing the second."

Thompson outlined the plans in a speech to the Royal Television Society on Wednesday, where he said he had already started to talk to independent producers and producers' trade association, Pact.

Thompson said the window would be "non-exclusive" and "open-ended – in other words, the programmes would be available permanently".

He added: "Our ambition would ultimately be to let everyone who pays the licence fee access all of our programmes on this basis and, over time, to load more and more of our archive into the window.

"It could also mark an important step in broadcast's journey from being a transitory medium into a growing body of outstanding and valuable content which is always available to enjoy and which persists forever."

Thompson, who is expected to step down as director general later this year or early in 2013, said he did "not propose to lay out an exact timetable this evening" about his departure.

"I'll share that with the BBC Trust and all of my colleagues at the BBC when the time is right."

But he did use the occasion to reflect on his eight years in charge of the corporation and to take aim at his critics in parliament and the press.

On his decision to shut 6 Music, which was reversed following an outcry from listeners, Thompson criticised MPs who had "been most vocal about the need to cut BBC services [who] promptly turned on a sixpence" when the public came out in support of the station.

He did not name names but broadcasting minister Ed Vaizey, then in opposition, springs to mind.

"It's been much the same with the Asian Network and with the more recent debate about sharing some programmes on local radio," added Thompson.

On "Crowngate", the misleading footage of the Queen which forced the resignation of the then BBC1 controller Peter Fincham in 2007, Thompson said: "The splash in that morning's Times was CRISIS OF TRUST AT THE BBC in a font size which a short-sighted mole could have read at 20 paces, but which seemed to take a little longer to find when that newspaper reported its own rather more recent computer-hacking scandal."

He said the incident and another BBC "gate" – "Sachsgate" – were "serious but isolated mistakes in what are usually highly dependable services with exceptional standards and values".

"We were heavily criticised for being too slow to respond to The Russell Brand Show," he said.

"Yet within four days of the story breaking in the Mail On Sunday, we'd completed an investigation into what had happened, two senior editorial leaders had left the BBC, one presenter had resigned while the other had been suspended, and we'd announced how we intended to ensure there would be no repetition of such a failure.

"Compare those four 'slow' days with the long years of phone hacking."

Thompson also defended the pay bill of the BBC's senior management, including himself, which he said amounted to "less than 2% of the BBC's costs".