The Scottish National Party has become the UK's third largest political party following a huge surge in membership since the Yes campaign's defeat in the independence referendum.

According to the party's chief executive Peter Murrell, more than 26,000 supporters joined its ranks over the last four days, taking the number carrying a party card from under 26,000 to more than 51,000 - doubling its membership.

It would represent an extraordinary response to events last Thursday and the resignation of party leader Alex Salmond the following day.

It also means that the SNP membership, drawn from a tenth of the UK population, now outnumbers the Liberal Democrats in terms of card-carrying membership.

Nationalist MSPs, gathering again at Holyrood as the Parliament reconvened, were bullish about the aftermath of the referendum and where Scottish politics goes next in the wake of the 55 per cent to 45 per cent defeat for the Yes side.

"Devolution was meant to kill nationalism stone dead," said one MSP. "Then the referendum and the resignation of Alex Salmond too. It doesn't seem to happen, does it?"

It is claimed that 3,000 new members have joined the Scottish Greens while the Scottish Socialist Party, the other party in the Yes alliance, also claims a surge in interest in membership.

The number of SNP members was said by Murrell to be 50,000 at 12.40pm today, compared to 25,642 last Thursday.

The last UK-wide LibDems' membership number was 43,451 at the start of this year, but the party said it expected to top the 44,000 mark in April. Ukip had 38,124 members as of May 19.

SNP business convener Derek Mackay said: "Scotland's referendum was an incredible triumph of democracy and the new wave of democratic engagement and activism the Yes campaign inspired shows no signs of stopping. That over 22,000 new members have joined the SNP in the last few days is incredibly encouraging and confirms that it is the SNP that the people of Scotland trust to work in Scotland's best interests."

He claimed Labour's position in Scotland was growing "more precarious" as he called on the Westminster parties to fully deliver on their vow for more powers for the Scottish Parliament.

In Manchester, Ed Miliband will today tell delegates at the Labour conference: "We will show the people of Scotland over the coming years that they made the right choice; because we are better together."

In his keynote speech, the Labour leader will acknowledge that Britain nearly broke up and that a country that almost fractures "is not a country in good health". He will use a campaign anecdote about Josephine, an office cleaner, who did not want to vote Yes but thought, given the hard times she was suffering, it might be the best thing to do.

"I hear Josephine's question everywhere, not just in Scotland," Mr Miliband will say. "Can anyone build a better future for the working people of Britain? It wasn't just the referendum question. It is the General Election question."

Also in his speech, the Labour leader will set out a 10-year mission if he wins power to "restore faith in the future and ensure the next generation does better".

Yesterday, Alistair Darling responded to Alex Salmond's suggestion Scottish independence might be achieved through a "parliamentary route".

The former Chancellor, who led the Better Together campaign, said: "I just say to him: you lost the argument, you lost the referendum, you've lost office and now you've lost the plot. The people of Scotland are sovereign and the people of Scotland have said 'no thanks' to separation and you must accept that result."

Meanwhile, Shadow Chancellor Ed Balls would be "staggered" if Gordon Brown were to want to become First Minister. He told a Fringe event in Manchester: "I have huge respect for the Scottish Parliament but being First Minister in Scotland isn't the same job as being the UK Prime Minister."