Former Conservative Party chairman suggests a Tory-Labour grand coalition may be needed to stop the SNP

A former chairman of the Conservative Party has suggested a Tory-Labour grand coalition could be needed after the election to stop the SNP holding the balance of power at Westminster.Lord Baker of Dorking said a deal between the two largest parties could be necessary to ensure the "continuing unity" of the UK.

The former education secretary said such an agreement could avoid the "nightmare" of a minority Labour administration depending on SNP support to govern.

Writing in The Independent, he said a Labour-SNP pact at Westminster could "stretch the constitution of our country to breaking point".The former Cabinet minister accepted that a deal between David Cameron and Ed Miliband was "quite unthinkable" at the moment, but pointed out that in Germany Angela Merkel's Christian Democrats governed with the Social Democrats.

Lord Baker argues: "What is at risk is the continuing unity of the United Kingdom. In order to preserve that unity another way should be found.

"This could be a joint government of the Labour and Conservative parties: quite unthinkable at the moment, and at this time likely to be rejected by both of them - but this is what has happened in Germany."

Lord Baker's intervention follows a warning from former prime minister Sir John Major that the SNP would enter any deal with Labour with the "overriding aim" of "prising apart" the union.

In a sign of the influence the SNP hopes to wield after May 7, former first minister Alex Salmond has claimed Scotland will be able to "call the tune" at Westminster.

Mr Salmond, who is standing for the Gordon constituency in May, believes a large group of SNP MPs will lead to "progress for Scotland".