Another 20,000 soldiers could face the sack to help pay for the multi-billion pound Trident renewal, a former defence minister has warned.

Lib Dem MP Nick Harvey said officials were already drawing up proposals to slash the Army from 80,000 to 60,000 personnel.

Speaking in a debate in the Commons, Tory Defence Secretary Michael Fallon also accused Labour of preparing to trade the UK's security for power by entering government with the SNP.

The SNP has said scrapping Trident would be one of its key demands if there is a hung parliament in May.

The debate against renewing the ageing nuclear weapons system on the Clyde was organised by the SNP, Plaid, and the Greens.

Mr Harvey said the Ministry of Defence faced a "financial crunch" and "already paper exercises (are)... looking at what an army of just 60,000 would look like".

He questioned why, under such severe pressures, Trident "gets an automatic bye".

That sentiment was echoed by former Labour shadow minister Diane Abbott who warned her constituents would "not understand" spending billions on a new generation of nuclear weapons at a time of austerity.

But Defence Secretary Michael Fallon defended Trident and accused Labour of being "prepared to trade our security if that was the price of power" and a coalition with the SNP.

In response Labour's shadow defence secretary Vernon Coaker told MPs his party remained committed to a continuous at sea nuclear deterrent.

However, most Labour MPs abstained on the vote, pointing to their party's policy to consider Trident as part of a defence review if it wins the election.

A new poll, timed to coincide with the debate, suggests that most Labour voters oppose Trident renewal.

Prof John Curtice, the polling expert, warned Trident and independence have become so intertwined it could make it more difficult for Labour to win back many Scots voters.

"(Labour) have to think if they are to get back ex-Labour voters who votes Yes, what they say about Trident could actually matter," he said.

But one Labour MP said that the party would be wrong to concentrate too heavily on the issue because polling also suggested that many voters who saw Trident as a top priority "are already voting SNP ".

The survey, carried out by pollsters Survation and commissioned by the SNP, also showed that most of those who expressed a view opposed Trident renewal.

Opening the debate the SNP's Angus Robertson said the case was "stronger than ever" to scrap Trident.

Getting rid of the nuclear deterrent on the Clyde would offer "serious strategic and economic benefits", he said.

And he warned that militarily replacing Trident was the "wrong priority", when money could be invested in other parts of the Armed Forces.

The UK, he argued, would be lauded internationally if it scrapped its nuclear weapons system.

Mr Fallon accused the SNP of wanting to "gamble" with the UK's security by proposing unilateral disarmament.

The Tory MP also compared Trident's Scottish home to Greenham Common, saying its deterrent effect mean Faslane was "truly... Britain's peace camp".

But he admitted that he could not say how much renewing Trident would cost until the so-called "maingate" decision next year, although he rejected the SNP's £100bn estimate.

Mr Fallon also announced that a ,target to cut the number of available nuclear warheads to 120 had been met.

And he hit out at Mr Harvey's Lib Demos, who support a scaled-back nuclear deterrent, accusing the party of wanting a "part-time" weapons system that involved the "pointless" unarmed patrols.