Boris Johnson has said he wants to raise the national insurance threshold to £12,500, letting slip a major Tory tax cut from the manifesto as he was speaking to workers in Teesside.

The prime minister blurted out the key announcement as he was pressed by an employee at a fabrication yard about whether he would help “people like us”, not just the rich.

However, it quickly emerged that the Tories would only pledge to raise the threshold to £9,500 next year, then lift it gradually over many years until it reached the target £12,500.

The policy would initially be a tax cut of about £85 a year and aim towards a reduction of around £460 for everyone earning more than the current threshold of £8,632.

Will PM's national insurance pledge help 'working people'? Read more

Johnson originally claimed the target would be £12,000, not £12,500, then stumbled over the timescale for the promise. He told one broadcaster it would be over the course of a parliament but it was later clarified to be a long-term ambition with no deadline. He also appeared to suggest wrongly that the initial £9,500 level would lead to tax bills lower by £500 a year, instead of £85 per year.

“The £9,500 … it’ll be soon in the next parliament if we’re lucky enough to be re-elected. This will put around £500 in people’s pockets,” he said, incorrectly.

The Institute for Fiscal Studies has calculated that the initial increase alone would cost £2bn. Xiaowei Xu, a research economist at the IFS, said reducing the NI threshold was the “best targeted way of helping low earners if restricting oneself to the direct tax system”, but added: “If the intention is to help the lowest-paid, raising the NICs threshold is an extremely blunt instrument. Less than 10% of the total gains from raising NICs thresholds accrue to the poorest fifth of working households.”

Johnson was caught off-guard by the question from Claire Cartlidge, a fuel quality manager at the yard, who asked whether he was promising low taxes for all or just for people like himself.

“I mean low tax for working people,” he said. “If you look at what we are doing and what I said in the last few days, we are going to be cutting national insurance up to £12,000. And we are going to make sure we cut business rates for small businesses, we are cutting tax for working people, by the way.”

Tory advisers were taken by surprise by Johnson letting slip the announcement, which was due to be a flagship element of the manifesto briefed to Sunday newspapers.

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The manifesto is due to be published on Sunday, which Tory sources said was aimed for maximum impact on the weekend evening news bulletin and to “attract the Countryfile audience” – referring to those who might switch to the news after watching a popular Sunday night programme.

However, the mistaken announcement will not be unhelpful to the prime minister on a day when his campaign has been overshadowed by a row about the Conservative party Twitter account masquerading as a factchecking service. Twitter has criticised the party’s move as misleading and warned it not to do it again.

Q&A How did the Tories pretend to be a factchecking service? Show Hide During the ITV live leaders debate on 19 November, the Conservative party re-branded their press office account on Twitter as 'factcheckUK', to tweet anti-Corbyn points during the programme to its 75,000 followers. On Twitter accounts there is a username - in this case @CCHQpress - and a screen name, which appears more prominently. The Conservatives changed the screen name to 'factcheckUK', and also changed the logo and biography of the account to read 'fact checking Labour from CCHQ'. No explicit mention of the Conservative party name in full was made, so users would have to know that CCHQ is an acronym for 'Conservative campaign headquarters' in order to understand who was providing the fake fact-checking service. Because the @CCHQPress account on Twitter is 'verified', it means when it appears it has a blue check mark next to the name, to show that Twitter has 'verified' that the account is who it says it is. This was retained while the account was tweeting under the false name 'factcheckUK'. Martin Belam

Earlier on the campaign trail, Johnson said his manifesto would tackle the social care crisis and repeated a pledge to make sure no older person would lose their home in order to fund their care. There were reports on Wednesday night that he plans to put an extra £5bn of funding into the system.

Other staff at the energy firm pressed the prime minister on the social care funding crisis and whether Jeremy Corbyn was right to say he would sell off the NHS in a trade deal with the US.

One worker also asked him why he would not reveal the contents of the intelligence and security committee report on Russian interference in the EU referendum. Johnson said he had “seen no evidence of interference” by Moscow in Brexit and insisted he was sticking to the original timetable for the release of the report, despite calls from Dominic Grieve, the committee chair, for it to be published before the election.

The prime minister was later grilled by journalists on his manifesto but he declined to give any more details.

Asked whether he would release his own tax return, he said he “always” released his tax return, even though he has not done so since he was London mayor in 2016.

However, he did answer a question about the HS2 high-speed rail line from London to the north of England by hinting that he would proceed with it, despite having ordered a review of its affordability.

“You know where my instincts are. I’ve overseen a great number of very big infrastructure projects,” he said. “I’m going to hesitate before simply scrapping something that has been long-planned and is of great national importance.

“But we will want to be checking the money is being properly spent and there aren’t ways in which it could be reprioritised or reprofiled.”