SNP supporters burst into applause when Nicola Sturgeon told them the party "will always support independence".
SNP plans to abolish air passenger duty in Scotland could impact on airports in the rest of the country, David Cameron said.
Speaking in Crewe, Mr Cameron said: "The SNP government in Scotland is committed to using its new powers to cut and eventually abolish air passenger duty for flights from Scottish airports
"That could distort competition and see business drawn north of the border with a huge impact on airports in the rest of our country so we're reviewing the way air passenger duty works to make sure other cities don't loose out".
Announcing the Conservatives 'Carlisle Plan', Mr Cameron said a Conservative government would conduct an annual review of the impact of all devolved policies on England, Wales and Northern Ireland.
"This is not about a UK government stopping the Scottish government from using its powers as it sees fit to do things differently, he said.
"This is about making sure we understand the impact that devolution is having and making sure the rest of the country never unwittingly loses out."
SNp leader Nicoa Sturgeon warned supporters the media were at the manifesto launch "to do a very important and legitimate job".
ITV News Deputy Political Editor Chris Ship reports from the SNP's manifesto launch:
The SNP would vote to increase the minimum wage to £8.70 by 2020, leader Nicola Sturgeon said at the party's manifesto launch.
Sturgeon said her party would crack down on zero-hours contracts, vote to restore the 50p tax rate and back the mansion tax.
Nicola Sturgeon was met with cheers when she told the SNP manifesto launch the party would always support independence.
Nicola Sturegon has vowed the SNP will not join a coalition with the Conservatives.
"We would not do any deals that would put the Tories into power," she told the crowd at the SNP's manifesto launch.
"Indeed if there is an anti-Tory majority after May 7, we will vote to stop a new Tory government even getting off the ground."
Sturgeon said her party would seek to make a Labour government "bolder and better", using their eight years experience of being in power in Scotland.
Nicola Sturgeon unveils the SNP's manifesto at the party's election launch.
"This is a manifesto to make Scotland stronger at Westminster", she told the crowd in Edinburgh.
Crowds are gathering for the SNP's election manifesto launch at the Edinburgh International Climbing Centre.
Distraught relatives of the Muslim family who are believed to have left England for Syria have begged them to come home.
In a statement released through Thames Valley Police, the family spoke of their "complete devastation", and said the six "left the UK without any notice".
We are totally distraught, upset and in shock. The greatest sadness is the decision by Asif and Sara to travel with their young family, such beautiful children.
Please can you ring us or send us a message to let us know that you and the children are well. All we wish for is your safe return and especially the return of the children. We are not angry with you, we are just heartbroken.
Asif Malik, his partner Sara Kiran, and his four children aged between seven and one, vanished from their Slough home earlier this month, and were seen travelling through Europe by train.
The Lib Dems pledge to create an independent education standards watchdog to restrict ministers' ability to interfere with the curriculum.Read the full story ›