David Cameron and Ed Miliband shared a rare moment of unity in the Commons on Wednesday as they pledged to fight for the future of the United Kingdom.

After the threat of an independent Scotland was raised at PMQs the prime minister said he was "100%" in agreement with the Labour leader that the UK was "stronger together."

"Frankly I'm sad that we are even having this debate because I support the UK so strongly," Cameron told MPs.

And looking to the SNP benches, the prime minister said: "I look forward to having the debate because I think there are too many in the SNP who are happy to talk about the process but not about the substance.

"I sometimes feel that it's not a referendum they want, it's a never-endum."

He added: "I passionately believe in the future of our United Kingdom. I passionately believe we are stronger together rather than breaking apart."

Miliband said his party believed the UK benefitted people in Scotland and the rest of Britain "in equal measure".

"We are stronger together and weaker apart.

"We must make the case for the Union - not simply against separatism, but the positive case about the shared benefits to us all of Scotland's part in the United Kingdom, the shared economic interests, the shared institutions ... and all the shared values we hold together."

But Alex Salmond accused Westminster of trying to interfere in Scottish affairs, and said Cameron should "butt out", and said the prime minister was being Thatcher-esque.

"The prime minister came in with his size 10 boots and started to put all sorts of strings and conditions, and basically wanted to pull the strings of Scotland's referendum."

Salmond said there had been a "huge adverse reaction" north of the border to "the Thatcheresque idea that Downing Street knows best".

SNP MP Angus Robertson said Miliband had "deepened the Labour split" by sharing a platform with the Tories.

"Labour will be misreading and misrepresenting the Scottish people and many in their own party if they continue with this plan to team up with the Tories."

Cameron and Miliband's comments came after Alex Salmond struck a slightly more conciliatory tone regarding a Scottish independence referendum on Wednesday morning.

The SNP first minister suggested there could be further negotiations with Westminster on the mounting constitutional row.

But Salmond insisted there was a case for Scotland being able to hold a referendum without asking London first, and hasn't relaxed his view that the vote should be held in the autumn of 2014.

"The objection is that the prime minister suddenly this week decided to start pulling strings and setting conditions," he told BBC Radio 4's Today programme.

The coalition government admits it is in untested territory, but believes a referendum could be held as early as a year from now.

Ministers have drawn up what's called a Section 30 Order, which would allow Scotland to hold a referendum without asking Westminster for permission, however this order needs to be agreed by both the parliaments in London and Edinburgh first.

However it has attached a rider to the order, suggesting a deadline for the referendum should be agreed before the law is changed.

Scottish ministers have not set out what question - or questions - will be put to voters.

Salmond's spokesman said the "preference" was for a yes or no question on independence, but added they were "entirely open" to people also being given the option to vote for greatly enhanced powers for Holyrood.