Legalised assisted dying should “gradually” be extended to dementia sufferers, the author Ian McEwan has said.

McEwan was “shocked by the snow-drilling attempts” by those opposed to the UK’s assisted dying bill, he told a public book event in London, citing its more than 1,000 amendments. MPs and peers backing the bill now believe it is “near impossible” for it to pass the House of Lords before the end of the session in May due to alleged filibustering.

If passed, the bill would legalise assisted dying in England and Wales for adults with less than six months to live. “We’re not asking much,” said McEwan, who is a patron of Dignity in Dying.

“I like it when some bishop says on the radio: ‘It’s the thin end of the wedge,’ and I think yes, it is the thin end of the wedge”, because certain groups are “missing from it”, such as those with dementia. “It has to be physical pain”.

“My guess is that if we pushed it through with all the protections around it – of doctors and dispassionate people making judgments – we’ll look back on this and think, ‘Why did we ever let people die in agony?’”

Asked explicitly if he would add an amendment to extend assisted dying to dementia sufferers, McEwan said: “Gradually, yeah, I would. But I think it does require a lot more thought and the idea of living wills.”

“My mother used to say to me: ‘If I ever become really terrible, I’d like you to finish me off.’ But of course, that’s to commit murder as things stand. Imagine standing up in court and saying: ‘Well, she did say when we were on the beach 20 years ago …’”

McEwan spoke about dementia’s impact on his family – his mother Rose had dementia, as well as his brother-in-law and another close family member. “By the time my mother was well advanced and could not recognise anyone, she was dead. She was alive and dead all at once. It was a terrible thing. And the burden on those closest is also part of the radioactive damage of it all.”

McEwan was speaking at St Martin-in-the-Fields church in central London, as part of its Conversation series, discussing his latest book, What We Can Know, in which dementia is a major theme. He has also written about dementia in previous novels, Lessons and Saturday.

The Atonement author also discussed the new novel he is working on, during a conversation about the ban on social media platforms for under-16s in Australia, and the measure’s potential adoption in the UK. The author said that he is beginning “to wish that the internet didn’t exist. I think back enviously to the 70s, where one of the great luxuries of civilisation – which is solitude – was bounteously available, and has been worn away, and now so much dark stuff is coming out of it. I’m trying to write a novel about this. The disappearance of childhood, or the sense of childhood being under escort.” He said that he was in favour of social media bans.

A major theme of What We Can Know is climate change: the novel is partly set in 2119, at which point Britain has become an archipelago, having been submerged by rising seas. McEwan said that while he has “never known the world in a worse state”, he maintains a “little streak of optimism that we’re going to scrape through”.

What We Can Know has an “emotional background” made up partially of despair, and partially of hope, said the author. If you have children and grandchildren, “you want the human project to survive”. Yet, there’s a “countervailing current” common to all old people, he said – in order to make sense of their lives, people think: “with the end of me, it’ll be the end of everything – ‘après moi, le déluge.’ Elderly pessimism is a very powerful constraint on clear thinking”. He sees What We Can Know as an expression of those “contrary forces”.