Voters have been left with an "unacceptable" level of uncertainty about political parties' positions on the national deficit, with the formerly dominant issue "falling off the radar" of the election debate, a think tank has warned.

The scale of the deficit dominated campaigns in 2010 and 2015, as rival fiscal targets set the framework for a sharply differing visions on spending, taxes and austerity.

But with just four weeks to go to the June 8 election, the Resolution Foundation think tank said that "huge uncertainties" remain about the main parties' fiscal stances - which have scarcely featured in the election debate.

The gap between Conservative and Labour consolidation plans could be as narrow as £7 billion, or as large as £50 billion or more, the foundation said in a report entitled "The Deficit the Election Forgot".

Tories would need to find tax rises or spending cuts totalling £17 billion by 2021/22 to meet their "fiscal objective" to achieve an overall budget surplus by the end of the next Parliament, as set out in Chancellor Philip Hammond's 2016 Charter for Budget Responsibility, said the foundation.

But the party's "fiscal mandate" to reduce cyclically adjusted borrowing below 2% of GDP by 2020/21 - set out in the same document - would allow for £30 billion of fiscal loosening by the end of the next Parliament, granting the Chancellor substantial leeway to be generous.

Meanwhile, the foundation said that Labour's "fiscal credibility rule" of a current budget balance in five years would allow John McDonnell to loosen the current budget plans by up to £37 billion by 2021/22.

This would provide a Labour chancellor with "more than enough" flexibility to cancel departmental spending cuts of £3.5 billion and welfare cuts of £8 billion, announced by the Government but not yet implemented, said the think tank.

But it warned that using up all this headroom would result in a slower reduction in debt than currently projected.

Matt Whittaker, chief economist at the Resolution Foundation, said: "Labour and the Conservatives went into the 2015 election with fiscal plans that were £30 billion apart.

"Two years on, the uncertainty surrounding their plans means that gap could have narrowed to just £7 billion, or widened to over £50 billion.

"This scale of uncertainty is unacceptable - voters deserve more clarity than they're currently getting from the main political parties.

"With just four weeks to the General Election, the forgotten deficit should reappear in the parties' manifestos next week."