Boris Johnson has been criticised for misleading voters over the Tories’ record on the NHS, after he sent letters to voters in swing seats that selectively quoted a charity.

The letter, sent out across marginal seats such as High Peak and Reading West, highlighted comments from various media sources and charities praising the long-term plan for the NHS, which was set out before Johnson took office.

One of the highlighted quotes, from Sarb Bajwa, the chief executive of the British Psychological Society, lauded the plan for a “clear commitment to mental health through increased spending and introducing access standards”.

But Johnson’s letter missed off subsequent parts of the quote saying that there was “still a long way to go”, though the plan showed the NHS was listening to concerns about mental health provision. It also highlighted the need for “immediate action for children and young people’s services as they have become woefully underfunded and overstretched” and for mental health provision outside the NHS to be resourced effectively.

A BPS spokesperson said: “The BPS works to promote excellence in psychology and support the psychological workforce. We remain entirely politically neutral and were unaware that this comment had been taken from our website and used out of context as a political endorsement.”

Another quote on the letter comes from the BBC website, showing just the headline: “NHS long-term plan: Focus on prevention ‘could save 500,000 lives’.” The article refers to concerns from unions that staffing shortages could undermine the ambitions and a comment from Dr Nick Scriven, the president of the Society of Acute Medicine, saying he was “staggered” that the plans overlooked acute and emergency care and hospital capacity.

The British Heart Foundation, a charity also quoted in the letter, said it had not been consulted about its inclusion on the election material, but it would work with any government of the day on the NHS long-term plan, which it supported.

Jonathan Ashworth, Labour’s shadow health secretary, said: “The truth is the Tories have spent a decade running down our NHS, cutting over 15,000 beds, starving the NHS of cash, privatising key services and failing to recruit the extra nurses, doctors and staff needed.

“They now even risk the NHS being sold off in a Trump trade deal that will cost our NHS £500m a week. No one will believe this desperate spin from Boris Johnson.”

The government has denied that the NHS is any way “up for sale”, and said it “could not agree to any proposals on medicines pricing or access that would put NHS finances at risk or reduce clinician and patient choice”.

Johnson has put the NHS at the centre of the Conservatives’ campaign, despite Labour traditionally being more trusted on the health service and the likelihood that the government will be blamed for any winter crisis that emerges as a result of squeezed services.

Tory MPs are nervous about the move, but the party’s strategists believe Johnson is associated by voters with prioritising the NHS because of the highly controversial Vote Leave pledge to spend £350m a week on the health service after Brexit.

On Monday, Ashworth released research showing the number of operations cancelled due to staffing issues and equipment failures in the health service had increased by a third in two years. The Conservatives contested the way he presented the information, saying Labour’s own freedom of information figures showed that the number of cancelled operations overall fell last year.

Earlier, Chris Hopson, the head of the organisation representing NHS trusts, called for parties to avoid “cheap political slogans” about health during the general election campaign, and instead engage in a rational debate about the best way to fund and manage the service.

The chief executive of NHS Providers stressed that his organisation was politically neutral and not seeking to criticise any party. But pointing to examples of potentially misleading campaigning, Hopson noted that the level of NHS funding increases promised by the Conservatives would do no more than maintain current provision, rather than take the NHS to “sunlit uplands”.

Asked about Labour’s warnings on NHS privatisation, Hopson said the debate on outsourcing had to be framed around the point that some private provision was seen as working well; for example, hospices.

“What really worries us, if you look back over the last four or five elections, the debates about our health and care systems have generated extreme amounts of heat and very little light,” he told BBC Radio 4’s Today programme.

“What we’re very keen to do is ensure that as we start to think about that debate in the general election campaign, we frame it in the right way.”