Politicians must get off the fence on the issue of assisted dying, broadcaster Jonathan Dimbleby has said as MPs began a lengthy debate on the law at Westminster.

Campaigners both for and against legal reform gathered outside Parliament on Monday afternoon after a petition backed by Dame Esther Rantzen gathered more than 200,000 signatures.

Three hours have been set aside for the Westminster Hall debate, at which various MPs can air their views on whether or not they feel a change in the law is necessary, but there will be no vote at the end.

Get off the fence, don’t sit on your hands, have a proper full debate about all the implications, and at the end of that I am sure they will introduce legislation

A number of high-profile figures have spoken out on the issue in recent months.

Dame Esther, who has stage four lung cancer, is not attending due to her health but has vowed to watch the debate closely, saying a change in the law “would mean that I could look forward in confidence to a death which is pain-free surrounded by people I love”.

Her efforts in speaking out on the issue have been praised by fellow pro-change campaigner Dame Prue Leith, who cannot make the debate due to filming commitments but has called for “less pearl-clutching” and more “serious, constructive debate”.

Dimbleby, who gathered with pro-change campaigners on Monday, has previously described the current law as “increasingly unbearable” following the death of his younger brother Nicholas, who suffered with motor neurone disease (MND).

In a message to MPs, he told the PA news agency: “Get off the fence, don’t sit on your hands, have a proper full debate about all the implications, and at the end of that I am sure they will introduce legislation.”

He described the Not Dead Yet protest being held next to the Dignity in Dying demonstration as impassioned but unreasonable, saying some of the slogans were “scare stories that I wish that people wouldn’t deploy because of their own very strong feelings”.

Instead of discussing this dangerous and ideological policy, we should be talking about how to fix the UK’s broken and patchy palliative care system so everyone can have a dignified death

Those who oppose a change in the law have voiced concerns that legalising assisted dying could put pressure on vulnerable people to end their lives for fear of being a burden on others and argue the disabled, elderly, sick or depressed could be especially at risk.

Dr Gordon Macdonald, chief executive of the campaign group Care Not Killing, described Monday’s debate as a missed opportunity to talk about fixing the UK’s palliative and social care system.

He said: “Instead of discussing this dangerous and ideological policy, we should be talking about how to fix the UK’s broken and patchy palliative care system so everyone can have a dignified death.”

The issue was last voted on in the Commons in 2015, when it was defeated at second reading stage by 330 votes to 118.

Opening the Westminster Hall debate, Labour MP Tonia Antoniazzi, a member of the Petitions Committee, said: “The fact that in less than two years we have had two petitions debates on this subject clearly indicates to me and to others that this issue is one our constituents are highly engaged in.”

She added that “whatever our own views we must recognise that public opinion on assisted dying has shifted in one direction”, citing polls by Dignity in Dying showing “overwhelming support for law changes with safeguards in place” and a rise in UK members of Dignitas.

She said Dame Esther’s public speaking on the issue had “contributed to a heightened awareness of assisted dying”.

Assisted suicide is banned in England, Wales and Northern Ireland, with a maximum prison sentence of 14 years.

In Scotland, it is not a specific criminal offence but assisting the death of someone can leave a person open to being charged with murder or other offences.

A Bill was introduced in Scotland in March – the third time members of the Scottish Parliament will have considered the issue – with two previous attempts to change the law defeated.

A report by MPs at Westminster in February warned that the Government must consider what to do if the law is changed in part of the UK or on the Isle of Man or Jersey, both of which are crown dependencies and both of which are currently considering the issue.

In a response published on Monday to the Health and Social Care Committee report, the Government said it would discuss with the devolved administrations and crown dependencies “the practical implications for England and Wales of legislation introduced to allow AD/AS (assisted dying/assisted suicide) and any constitutional issues that such legislation may present”.

While the Government said it would “expect the implementation period to allow time for such discussions whilst the necessary regulatory measures are put in place”, committee chairman Steve Brine said “conversations need to start earlier than that”.

Sir Keir Starmer has previously said he is “committed” to allowing a vote on legalising assisted dying should Labour win the general election while Downing Street has previously said it would be up to Parliament whether to debate legalising assisted dying again.