Boris Johnson has said it is “absolutely not true” that he misled the Queen over his reasons for suspending parliament, after a Scottish court ruled his suspension unlawful because he did it to stop MPs scrutinising Brexit.

The prime minister previously insisted he sought the suspension so that the government could set out a new legislative programme in a Queen’s speech on 14 October, but the court said the prorogation was obtained for the “improper purpose of stymying parliament”.

On Thursday, in his first comments since the judgment, Johnson said the high court in London had taken an opposing view and it was for the supreme court to make a final adjudication.

Brexit: Boris Johnson denies lying to Queen about reasons for prorogation - live news Read more

Asked whether he had lied to the monarch in order to obtain the prorogation, Johnson replied: “Absolutely not.” He added: “The high court in England plainly agrees with us but the supreme court will have to decide. We need a Queen’s speech, we need to get on and do all sorts of things at a national level.”

However, the London court, on 6 September, had made no ruling on the truthfulness of his reason for the prorogation, simply saying it was a lawful move because suspension of parliament was within the prime minister’s powers regardless of motive.

Johnson also sought to play down an argument about the role of Scottish judges in ruling prorogation lawful after Kwasi Kwarteng, the business minister, claimed “many people” thought judges were biased in relation to Brexit.

Opposition parties continue to press Johnson for a return to parliament, not least to scrutinise the possible effects of a no deal Brexit as revealed in the official Operation Yellowhammer documents released by the government on Wednesday.

MPs secured the release of the papers through the use of a humble address in the Commons but cannot now force the publication of other documents because parliament is not in session.

Johnson tried to minimise the significance of the paper, saying it represented a “worst-case scenario” and ministers had been “massively accelerating” their no-deal preparations since he entered No 10 in July.

“If we have to come out on 31 October with no deal we will be ready and the ports will be ready and the farming communities will be ready, and all the industries that matter will be ready for a no-deal Brexit,” he said. “What you’re looking at here is just the sensible preparations – the worst-case scenario – that you’d expect any government to do.”

Some MPs believe Johnson is moving closer towards trying to achieve a Brexit deal after realising the scale of disruption associated with leaving the union without an agreement. However, securing a deal would probably involve having to disappoint hardline Eurosceptics in his party.

Johnson said on Thursday: “I’m very hopeful that we will get a deal, as I say, at that crucial [EU] summit [in mid-October]. We’re working very hard. I’ve been around the European capitals talking to our friends. I think we can see the rough area of a landing space, of how you can do it. It will be tough, it will be hard, but I think we can get there.”

The EU’s chief negotiator, Michel Barnier, said they were still waiting to see proposals from the UK side to resolve the fraught issue of the Northern Ireland backstop. “We are still ready to examine objectively any concrete and legally operational proposals from the UK,” he told reporters in Brussels.

Andy McDonald, the shadow transport secretary, said it was essential MPs returned to parliament so they could question ministers about how Johnson intended to secure a deal. “This is more like emergency planning for war or a natural disaster,” he told the BBC Radio 4 Today programme, discussing the Yellowhammer document. “We cannot minimise this. It does not get more stark.”

If Johnson does not reach a deal with the EU he is bound by parliament to request an extension to article 50, which he has repeatedly promised not to do.

On a boat trip on Thursday morningto mark London International Shipping Week, Johnson appeared to attempt to switch the subject away from Brexit by raising the prospect of a bridge between Northern Ireland and Scotland, saying it would be “very good” and estimating the project would cost around £15bn. He revealed his thoughts about the proposal – which has drawn scepticism from engineers – as he spoke to schoolchildren playing with a model container ship onboard the NLV Pharos, a lighthouse tender, on the Thames.

He told the children that he had recently been discussing the possibility of constructing a bridge over the Irish Sea. “I [was talking yesterday] about building a bridge from Stranraer in Scotland to Larne in Northern Ireland,” he said. “That would be very good. It would only cost about £15bn.”