Keir Starmer has accused Rishi Sunak of electioneering as the prime minister’s plans to detain and swiftly deport thousands of asylum seekers were greeted with scepticism from unions, refugee charities and some former Conservative ministers. As more people seeking refuge in the UK arrived across the Channel in chilly conditions on Monday, the Labour leader said the plans echoed previous announcements made to shore up support before local elections. The prime minister is to publish legislation on Tuesday which is supposed to give ministers powers to detain thousands of people and place upon the home secretary a duty to remove them “as soon as reasonably practicable” to Rwanda or another country deemed to be safe. One part of the proposals would set out that anyone who had crossed the Channel on a small boat would not just be removed to a third country but barred permanently from re-entry or ever applying for British citizenship. The proposals have been widely condemned by refugee groups and others as impossible to enforce and likely to lead to tens of thousands of people fleeing war and persecution being locked up. Starmer questioned the timing of the latest attempt to tackle the Channel crossings, suggesting it was an electoral tactic for May’s local contests in England. “We had a plan last year which was put up in lights – ‘It’s going to be an election winner.’ These bits of legislation always seem to come when we’ve got a local election coming up,” he told LBC Radio. “It was going to break the gangs – it didn’t. Now we’ve got the next bit of legislation with almost the same billing. I don’t think that putting forward unworkable proposals is going to get us very far.” Starmer was referring to the government’s Nationality and Borders Act, last year’s attempt to tackle the problem by bringing in a two-tier system which reduces the support available to those seeking asylum by irregular means. In a further development, the home secretary, Suella Braverman, has vehemently denied claims that when she was attorney general in 2020, she advised against proposals to circumvent human rights laws. Informed sources have told the Guardian that when ministers were working on the Sovereign Borders Act which later became the Nationality and Borders Act, Braverman advised against attempting to find a way of sidestepping the European convention on human rights (ECHR). “Suella did not want to help on derogation of the ECHR. In fact she produced advice that said it was not possible and would be in breach of an international treaty. Now she seems to say it is possible,” a source said. But Braverman’s office has hit back at the claims, saying she was not in a position to create policy. A source said: “This is absolute drivel pushed by people who have no clue what they are talking about. The legal parameters at the time were clear. The then attorney general worked within those on behalf of the government of the day.” Downing Street has declined to give a timescale for Sunak’s pledge to stop small boats, acknowledging there will be legal challenges. The prime minister’s official spokesperson said: “Obviously we want to do this as quickly as possible. As we’ve always said, we recognise there will likely be challenges in many forms to this sort of legislation.” Lucy Moreton, from the Immigration Services Union, said legislation to remove people to a third country seemed “quite confusing”, given a plan to deport people to Rwanda was on hold amid legal challenges, and that arrangements to return refugees to other places in the EU where they had previously claimed asylum lapsed for the UK following Brexit. “So unless we have a safe third country that isn’t Rwanda to send people to, this just doesn’t seem to be possible,” she told BBC Radio 4’s Today programme. Announcing new measures against small boats would also, Moreton argued, “fuel the service” for people-smugglers, at least in the short term, who would tell would-be arrivals that they needed to travel soon. Sir David Normington, a former permanent secretary at the Home Office, said it was “highly doubtful” that people would stop arriving in small boats because it was illegal. “These are people many of whom are desperate. They have fled from persecution, and being told that there’s been a change in legislation in the British parliament, I don’t think is going to make a big difference to them,” he told the Today programme. The courts have rejected previous plans to deport to Rwanda those entering the UK on small boats, but No 10 and the Home Office are proposing to insert a “brake” on human rights legislation in an attempt to stop legal challenges. Campaigners are concerned the legislation will lead to the inhumane and costly detention of tens of thousands of refugees. The only safe routes available to those wishing to seek asylum in the UK are through limited schemes for Ukraine, Afghanistan and Hong Kong. Emma Stevenson, the deputy chief executive of Choose Love, a provider of grassroots aid to refugees, said: “Only last week, 59 people including children and a newborn died at sea after being forced into making an impossibly dangerous journey. “Rather than offering a genuine lifeline to prevent tragedy, the government is trying to bypass its responsibility to the European convention on human rights.” About 45,000 people crossed the Channel last year, while officials have conceded that more than 80,000 could enter the UK this year. Sunak has made “stopping the boats” one of his five key pledges before the next election. The bill will be published before a key summit between Sunak and the French president, Emmanuel Macron, on Friday. It is understood Sunak will seek a substantial increase in beach patrols to stop refugees leaving French shores.