A former Conservative minister who was one of the key architects of the “northern powerhouse” has questioned whether either of the main parties are genuinely committed to devolving powers out of Whitehall.

Jim O’Neill, a former Goldman Sachs banker who served as commercial secretary to the Treasury in David Cameron’s government, said the Conservative manifesto was “striking for having very little detail in it”.

Speaking before a speech on Wednesday urging Cumbrian politicians to push for a devolution deal, Lord O’Neill said: “It is disappointing that there is so little about devolution in it. At the same time, it is clear from Boris’s three speeches that he has given up north, if you can take anything he says seriously, [the Conservatives] seem as though they will be trying to pursue the agenda one would associate with the Cameron days.”

Boris Johnson has previously promised to “do devolution properly” and “maximise the power of the north with more mayors across the whole of the north”.

Asked whether he believed anything Johnson said, O’Neill replied: “I’m chastened by my 17 months as a minister … I’ve had quite a lot of conversations with his people about this stuff and they say all the right things but I’ve heard it before.”

It was after discussions with O’Neill and others that George Osborne came up with the idea of the northern powerhouse in 2014 while he was chancellor, leading to the introduction of elected mayors in city regions such as Greater Manchester, Liverpool and Sheffield.

After he was sacked by Theresa May when she became prime minister in July 2016, Osborne founded the Northern Powerhouse Partnership thinktank to continue to lobby for devolution. O’Neill joined the board after quitting the government in September 2016. He said May’s advisers Nick Timothy and Fiona Hill had “made it clear they wanted to kill anything to do with devolution”.

O’Neill, a cross-bench peer, was equally unimpressed with Labour’s manifesto, which promises to “decentralise decision-making and strengthen local democracy” and “re-establish regional government offices to make central government more attuned to our English regions”.

He said: “It almost seems to me that they have no desire to be in power because it’s hard to take it seriously because of the sheer vastness of it all. It’s a shame because within it there are some things that are highly sensible, particularly the plans for social housing. But it is impossible to know what they are serious about or what they would prioritise because there is so much and it’s not deliverable.”

O’Neill said that if within a month of a new government forming, the ruling party did not follow through with detailed plans for devolution, he would “stop wasting” his time “on this stuff”.