Britain slid closer to the abyss last night as Gordon Brown lobbed a hand grenade into the endeavours of David Cameron and Nick Clegg to form a government following the inconclusive result of the general election, writes John Hipwood.

The outgoing (sooner or later) Prime Minister has the outrageous front to believe that he can run the country for the next five months with a supine Clegg in tow while the Labour Party chooses yet another unelected premier.

In a cricket match, it’s easy to work out the result if one side scores 306 runs and the other 258, but not in the strange world occupied by Gordon Brown.

The men and one woman who did not stand in the television leadership debates will be scrabbling around, back-stabbing and dishing the dirt under the Brown plan in a desperate effort to become his successor. All in the national interest, of course.

Here we see the Brown/Mandelson axis in operation. Arch-fixer Lord Mandelson knows that Mr Clegg could not work for any length of time with the Labour leader, so he has told the PM he must go preferably in five months’ time if a deal with the Lib Dems can be sealed.

Futile

This was enough for the Lib Dem leader to open formal talks with Labour while continuing with probably futile discussions with the Conservatives.

That must mean that Mr Clegg had decided by yesterday afternoon that he could not sell what was then on offer from the Tories to his MPs and his wider party.

A “final offer” then emerged from the Conservative negotiators — the promise of a referendum on the alternative vote (AV) voting system, which retains an MP’s links with the constituency but barely goes a third of the way towards the Lib Dems’ ultimate goal of full proportional representation.

But the star of the early TV debates, who ultimately led his party to a miserable third place last Thursday, is playing a very dangerous game. If he tries to keep feet in both Tory and Labour camps, he could end up splitting himself right down the middle.

Does he really believe that the nation would stomach a losers’ coalition, which still wouldn’t have a majority in the House of Commons, which would eventually be led by someone who wasn’t considered as a prime ministerial candidate just six days ago on polling day and which is the brainchild of the unelected Lord Mandelson and the unelected former Liberal Democrat Lord Adonis?

Former home secretary John Reid, a wise old owl, summed it up last night when he said it was a “very bad mistake” for Mr Brown to propose a Lib-Lab coalition.

“I think it’s bad for the country, I think it will prove disastrous for both parties in it, and in fact I think it’s bad for Gordon as well,” he said.

Such a coalition would be “inherently unstable” because it would not enjoy a majority but would require the support of other parties, Mr Reid suggested.

Concessions

And where does the fast-changing situation leave Mr Cameron, who has up to now played a quiet, behind-the-scenes part in his attempts to seal a deal with Mr Clegg? He cannot keep offering concessions to the Lib Dems without leaving his party behind.

Mr Cameron has rightly put the economic crisis at the top of his priorities, and wants a stable administration backed by the Lib Dems to enable him to do that.

But pretty soon he’s going to have to give Mr Clegg an ultimatum: come with me and let the British people vote on whether or not to change the voting system, or join with the discredited Mr Brown and his unknown successor and risk derision from the public.

The second course would more than likely lead to another general election within months, and it would have to be run on the existing first-past-the-post voting system.

If the Lib Dems end up being blamed for causing an unwanted election, they could end up by being virtually wiped out as a parliamentary party. What price the “progressive coalition” then?

The choices

The latest dramatic developments at Westminster leave a bewildering variety of possibilities for the eventual shape of the government which might emerge from last week’s General Election.

Here are the most probable outcomes:

Labour/Liberal Democrat Coalition: This option looks more likely following yesterday’s developments. Mr Brown appears to envisage remaining as Prime Minister for about four months in such an arrangement, after which a new Labour leader would presumably become the second “unelected” PM in succession.

Labour and the Lib Dems cannot between them form a majority coalition. With a combined total of 315 MPs, they would need the support of a combination of parties in order to get the Queen’s Speech through the House of Commons.

An administration of this kind is unlikely to struggle on much longer than the few months it would take to install a new Labour leader.

Progressive Alliance: This option was floated by SNP leader Alex Salmond but has been played down by Labour.

Combining all the parties which regard themselves as progressive it could potentially command the support of 329 MPs, giving it an outright majority. However, each of the parties involved have tensions with one or more of the others, rendering such a wide-ranging pact potentially fragile.

In theory, an alliance of this kind could survive until the next expected election in 2014 or 2015, though it would suffer many perilous “wobbles” along the way.

Minority Conservative Administration: If Mr Brown is unable to command an authority in the House of Commons, he would go to the Queen and either suggest she ask David Cameron to do so or seek a dissolution of Parliament. At this stage the Queen would probably summon Mr Cameron.

If he has been unable to forge a formal coalition with the Lib Dems, Mr Cameron could seek a looser “supply and confidence” arrangement under which smaller parties would agree to back his administration in key votes.

While none of the other parties would wish to be branded “wreckers” by bringing down a minority administration, such an arrangement would be unstable and Mr Cameron is likely to seek the earliest favourable opportunity to call a fresh poll in the hope of securing a majority.

Conservative/Liberal Democrat Coalition: This seemed the most likely permutation for the new administration until yesterday’s developments.

A formal coalition, with Lib Dem ministers at the Cabinet table, would command a comfortable 78-seat majority. Although the parties would have to compromise on their policy agendas, they would be able to get a shared platform through the House of Commons without fear of defeat.

This would probably be the most stable of the potential solutions and could easily see out a full Parliament to 2014 or 2015.

Conservative Deal With Another Party: If the Lib Dems refuse his embrace, Mr Cameron could turn to the Democratic Unionist Party for backing. However, the DUP’s eight MPs would bring the voting strength of such an alliance up to no more than 315, meaning that support would also have to be sought from others.

This is a last roll of the dice for the Tories and may not even deliver enough voting muscle to secure approval of the party’s Queen’s Speech.

l No Deal: If neither Mr Brown nor Mr Cameron can assemble enough support to at least get their Queen’s Speech through the Commons and avoid a vote of no confidence, there is a real prospect of the Queen being left with no option but to grant a dissolution and send the parties back to the polls.

This outcome could result in a General Election as early as the summer.