Two coalition scenarios in the event of a hung parliament

Scenario 1

The SNP to Labour’s rescue

With Ed Miliband more than 30 seats short of an overall majority, the only way he can guarantee inheriting the keys to Downing Street is through a deal with the SNP. The phalanx of Nationalist MPs abandon the party’s usual practice of abstaining on English-only laws to ensure the minority Labour administration gets its budget and legislation through the Commons.

In return Nicola Sturgeon demands fresh and substantial devolution of powers. Her vehement opposition to the renewal of the Trident nuclear fleet on the Clyde provokes particular ill-will.

Meanwhile Tory MPs under the new leader, Boris Johnson, are in revolt over Labour pushing through policies in England for which it has no mandate.

Scenario 2

A Tory-Labour “grand coalition”

Once the shock waves subside from the SNP’s stunning electoral performance, senior Conservative and Labour figures start thinking the unthinkable: a deal to keep the Nationalists from power.

It is an almighty upheaval for parties who have spent the past century in mortal combat. Tory Eurosceptics are incensed that they will not get to vote on leaving the European Union. Some irreconcilables on both sides quit their party whip, but the grand coalition occupies more than 80 per cent of the Commons seats.

Work presses ahead on an overhaul of the UK’s constitutional arrangements, but the partners lack common ground on policy.

The grand coalition collapses in bitterness and acrimony, with a new election held in 2017.

Back in the real world, Labour and the SNP have refused to comment on scenario 1, while following the first calls for a 'grand coalition', Labour and the Tories have both flatly ruled out scenario 2.

Labour said it would "never, ever contemplate a coalition with the Tories", while the Tories said "we wouldn't endorse that idea at all".

More: [All the possible coalition combinations for next year's election]2#