david lammy , who is standing for london mayor , said the snp was a party labour could ` do business with ' in the event of another hung parliament .
the admission comes amid increasingly stark warnings from the tories that the snp will hold labour to ` lock out ' david cameron from number 10 in exchange for higher government spending , more taxes and increased borrowing .
former prime minister of ` threatening the integrity of the united kingdom ' by talking up the prospects of scottish nationalists .