The Green party would spend billions to give every adult – in and out of work – a citizen’s income worth more than £72.40 a week, the current value of the jobseeker’s allowance.

Natalie Bennett, the Green leader, said the policy would ensure no one lived in fear of losing their home or being unable to feed themselves. She said some of the cost would be recovered by the payment being withdrawn when an individual income reached a certain level.

Speaking on BBC1’s Andrew Marr programme, Bennett said the Green party, in common with the SNP and Plaid Cymru, would oppose the replacement of a Trident nuclear submarine, and would not form a coalition with any party; operating instead on a confidence and supply basis, leaving the party free to vote as it wished, case by case.

But with polls showing a hung parliament is the most likely outcome, Nick Clegg said he did not expect a result in which the largest party needed the support of any party other than the Liberal Democrats to form a government.

He said his party would not form a coalition that included Nigel Farage, and added he could not conceive of circumstances in which he would joined one with the SNP. He rejected suggestions that his parliamentary party would be cut in half.

Clegg said Ed Miliband was flaky on the economy and David Cameron was heartless about society, setting out how his party would act as a counterbalance.

The renewed focus on the cost and feasibility of a citizen’s income, including the way in which it would differ from the government scheme to integrate universal credit, demonstrates the extent to which Green policy is now being taken seriously. The payment would replace existing unemployment benefits, but would be paid to people with jobs as well as the unemployed.

Bennett defended the principle of the citizen’s income saying: “What it does is provide a sense of security. No one needs to live in fear. No one needs to think I have just been [benefit] sanctioned, I am not going to be able to put food on my table, I am going to lose my house next week”.

The cost of a citizen’s income has been put at £240bn, but Bennett said the payment would be withdrawn when an individual’s income reached a certain unspecified level.

She also said the party did not have an open-door policy on immigration, but those who have a right to immigrate should be entitled to exercise that right. She also questioned why Germany had been willing to take 20,000 refugees from Syria, and the UK figure was in the low hundreds.

She said public opposition to immigration stemmed from “low wages, expensive housing or crowded schools or hospitals – all these things are caused by failed government policies and not immigration”.