The Better Together campaign said the countdown to Scotland's independence referendum has begun after "months of unnecessary delay".
"We now have just over 18 months to go to win the arguments and win the referendum. We have already made huge strides ... However, we cannot for one minute assume that we have done enough", the campaign said on its website.
Former chancellor Ken Clarke has warned of the dangers of an economy based on oil revenues, as he attacked the SNP's predictions of a post-independence referendum North Sea boom.
Speaking in Edinburgh, Mr Clarke issued advice to Scottish First Minister Alex Salmond and Finance Secretary John Swinney not to "bet the ranch" on figures "pulled from the air".
Mr Clarke, UK Minister without Portfolio, said, "If we were to have an independent Scotland, it is not the case that it would not somehow have to face up to the consequences of economic crisis".
A full draft of the Scottish Independence Referendum Bill has been published on the Scottish Parliament's website:
First Minister Alex Salmond said he was "honoured" to announce Scotland's independence referendum will be held on 18th September, 2014.
"I believe it will be the day we take responsibility for our country, when we are able to speak with our own voice, choose our own direction and contribute in our own distinct way," Mr Salmond said:
First Minister Alex Salmond said the Scottish Independence Referendum Bill is "the most important legislation to be introduced" since the Scottish Parliament was established in 1999.
Mr Salmond said the legislation is important for what it would allow Scotland to achieve "with the powers of an independent country".
"Next year the choice facing the people is one of two futures. A 'no' vote means a future of governments we didn't vote for, imposing cuts and policies we didn't support", he told Holyrood.
"A 'yes' vote means a future where we can be absolutely certain, 100% certain, that the people of Scotland will get the government they vote for", he added
The Scottish independence referendum date has been welded in steel by apprentices at Steel Engineering Ltd.
Scottish Labour leader Johann Lamont said that with the date set "we can see an end in sight", but Scotland will be "on pause" until the independence referendum is out of the way.
The date means Scotland's independence referendum will be held in between the Glasgow Commonwealth Games and the Ryder Cup at Gleneagles.