10/03/2015 05:21 GMT | Updated 10/03/2015 10:59 GMT

SNP 'Screwing' UK For More Than Party Deserves, Says Baroness Shirley Williams

Matthew Lloyd via Getty Images
BRIGHTON, ENGLAND - MARCH 08: (L-R) Women and equalities minister Jo Swinson, Dame Shirley Williams and Deputy Prime Minister Nick Clegg talk before a speech during the opening day of the Liberal Democrat spring party conference on March 8, 2013 in Brighton, England. The conference opens in the wake of several scandals within the party, but also after a successful by-election campaign in the constituency of Eastleigh. (Photo by Matthew Lloyd/Getty Images)

The Scottish National Party is "screwing" David Cameron and Nick Clegg out of more for Edinburgh than it deserves given the party lost the independence referendum, Lib Dem Baroness Williams has warned.

In an interview with The Huffington Post, the former cabinet minister and veteran member of the House of Lords also said she did not think the Lib Dems would sign up to a second coalition with the Tories that included a commitment to hold an EU referendum.

Nicola Sturgeon has seen support for the SNP surge since its failed bid for independence in September last year and polls suggest it could obliterate Scottish Labour in May's general election. The SNP currently has six MPs, but some surveys suggest it could increase this to over 50. It is likely Ed Miliband will need to rely on the support of the SNP, in some form, in order to form a minority government.

Williams said Sturgeon and Alex Salmond were currently outmanoeuvring the coalition. "They are, at the moment, screwing both the parties in government for almost everything they can possibly get beyond what they are really entitled to," she said.

"They are asking for things they would be entitled to if they had won the referendum. Although they have got a remarkable machine, I accept that, and a remarkable campaign, what they haven’t been reasonable about is they can not expect to get everything form the UK that they would have got if they had won the referendum."

Williams also expressed concern about the "quite nasty streak in parts of the SNP" that was exposed during the referendum campaign. "It's not a tolerant party. They really tried to take journalists to the cleaners if they make any remarks unfairly of the SNP. That’s less true of the new first minister [Sturgeon] than it was of the previous first minister [Salmond] but its still quite serious. Not serious politically, but serious nationally and for the nature of the county and it worries me a bit," she said.

This weekend the Lib Dems will hold their pre-election spring conference in Liverpool. Despite abysmal poll ratings the party remains confident it will be able to cling onto around 30 MPs. The likelihood of another hung parliament also means the party may once again find itself in the position of kingmakers.

However Williams predicts that another Tory-Lib Dem would be "incredibly difficult" to form. "Not just for us. But awfully difficult for the Conservatives. Who can they work with? They can't work with the SNP. They can't work with Ukip. And if they work with us they can work with us up to the point where Europe emerges as a major issue. We're not going to give up. Their right-wing isn’t going to give up either," she said.

Clegg has staked out a position as the most pro-EU of all the major party leaders. However David Cameron has pledged that he would quit as prime minister if a second coalition did not deliver an in/out referendum by 2017.

Williams said any second Conservative-Lib Dem deal would depend on the "balance of power" within the Conservative Party. "We can work with the left-wing of the Conservative Party, the moderate wing of the Conservative Party. But we really can't work [with the right-wing]."

Asked if the Lib Dems should sign up to a coalition that had an EU referendum as part of the deal, Williams said she did not think the party would.

"The only thing we could possibly agree to, which I don't think we will, would be what happened with Harold Wilison," she said. In 1975 the then Labour prime minister lifted cabinet responsibility, allowing cabinet ministers to campaign on either side of that year's referendum on Britain's membership of the European Economic Community.

Williams said: "The absolute minimum would be a referendum that was completely open with no attempt to whip MPs and no attempt to try make the Lib Dems sign on. That's going to be quite difficult to achieve. I think there is almost everything to play for."