A last-minute all-party plan to devolve further powers to Scotland over tax, spending, welfare and a host of other areas will be unveiled in the next few days, George Osborne has said.

The announcement came after a YouGov poll showed the yes side taking a narrow lead, spreading fears at Westminster of a constitutional crisis.

The chancellor detailed the plans – the product of backroom talks – saying they would be unveiled in the next few days with a clear timetable for implementation in the event of a no vote. The offer is similar to the last-minute package offered to Quebec by the Canadian government in 1995, that staved off a vote for separation.

All the major political parties have presented different offers of further devolution, but there has been no agreement between them on far they should extend.

The announcement of the package also represents a shift away from the negative tone of the previous no campaign to a positive offer of the benefits of staying inside a United Kingdom. Many polls show support for wider devolution but there has been a lack of clarity about what it represents in practice.

The political parties said it was unlikely the joint statement would set out detailed new powers beyond those already promised, but focus on a credible timetable and process for the transfer to come about. The three parties have already issued a joint statement of further transfer of powers in June but, judging by the latest polls, this may have had little effect.

Labour – with most to lose from devolution – has arguably offered the least in terms of transferring tax-raising powers.

Osborne said it was clear Scotland wanted more autonomy and the Tories, Labour and the Liberal Democrats had agreed to deliver on that. "You will see in the next few days a plan of action to give more powers to Scotland. More tax powers, more spending powers, more plans for powers over the welfare state.

"That will be put into effect – the timetable for delivering that will be put into effect – the moment there is a no vote in the referendum. The clock will be ticking for delivering those powers, and then Scotland will have the best of both worlds."

He said the reforms would include much greater fiscal autonomy and control over tax rates as well as more powers over welfare rates, and "a host of other powers".

He said: "People should know if they vote no they are voting for more devolution, more powers for Scotland without the risks of separation. I think that is the best of both worlds."

But John Swinney, the Scottish finance secretary, said: "The only guarantee for additional Scottish powers for a Scottish parliament is to vote yes," adding that further powers had been promised by the Calman commission but were never implemented.

He rejected Gordon Brown's claims that for all the SNP language about greater fairness in an independent Scotland its chief commitment was a 3p cut in corporation tax.

Swinney pointed out that the shadow work and pensions secretary, Rachel Reeves, had promised Labour would be "tougher on welfare and tougher on benefits" than the current Westminster coalition government.

Osborne also played down speculation that David Cameron could be forced to resign in the event of a yes vote. "This is not a protest vote. This is a once-in-a-lifetime vote. This is not a vote about the next five years.

"This is a vote about the next 300 years. This is not about the future of the British government in Westminster. This is not about the future of myself or David Cameron or anyone else."

Osborne used the narrowing of the polls as a galvanising force, saying there was now no reason for any opponent of separation to stay at home on polling day. He said: "This country faces a very, very big choice. Scotland faces a very big choice.

"If people were in any doubt that they can stay at home, that they don't need to go out to the polls and vote no to avoid separation, they won't be in that doubt today.

"They should also be in no doubt about the consequences of this decision – one of which is that Scotland will not be sharing the pound as an independent country with the rest of the UK if the separatists win the vote."

Osborne said Alex Salmond, the SNP leader, was not telling people the straight truth about what kind of currency Scotland would use. "What are the notes I am going to pull out of my wallet or coins out of my purse, and the coins I have in my pocket if this country is separate from the rest of the UK?"