Jeremy Corbyn's approval rating has fallen to -39 – and Labour voters are more critical of his leadership than of Ed Miliband's

Jeremy Corbyn puzzled the media at the weekend by suggesting Britain's Trident nuclear submarines "don't have to have warheads on them". Since 1969 Britain has had a submarine carrying nuclear weapons on patrol somewhere in the world's oceans, to deter enemies even if the nation's defences are destroyed, but Mr Corbyn seemed to propose these could carry conventional weapons in order to mitigate job losses in the defence industry. A decision on Labour's nuclear policy will likely be announced in the summer, but when other issues like the doctors' strike could be ammunition for the Labour party critics are angry about the distraction.

New YouGov research conducted prior to Corbyn's Trident claims reveals his net approval rating has fallen a further seven points to -39 since December 18, now only two points above his all-time low of -41 on December 1. David Cameron's meanwhile is at -6; below his post-election average of +3 but above his May 2010 – May 2015 average of -9.

Among the general public there is a strong tendency to believe Jeremy Corbyn has changed the Labour party for the worst since his election. 45% say he has had a negative effect on Labour compared to 21% who say he has changed it for the better. On September 28 2011, a few days after Ed Miliband became leader, few said he had changed Labour for the worst (11%) and most people (56%) said he had made no difference.

Labour voters are more likely to say Jeremy Corbyn has had a positive effect (37%) than a negative effect (27%) on the party, but are still more likely to be critical of Corbyn's effect on the party than of Ed Miliband's at the early stages of his leadership (36% thought he'd improved Labour, while only 5% thought he'd hurt it).

In spite of this, most people (55%) who voted Labour in 2015 now say it's unlikely they will win the next general election with Jeremy Corbyn as leader while only 27% say it is likely they will. This is a significant increase in pessimism – in September 41% of Labour voters said it was unlikely Labour would win under Corbyn and 35% said it was likely.

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See the full poll results