FORMER SNP deputy leader Jim Sillars is facing the threat of expulsion from the party after campaigning for a left-wing rival.

The veteran Nationalist has sparked anger among party loyalists by appearing in a campaign video for RISE, the socialist alliance hoping to win regional Holyrood seats at the SNP's expense in the election in May.

The SNP's Spring conference starts today in Glasgow, where the most prominent campaign slogan will be "Both Votes SNP".

Nicola Sturgeon will use her party's biggest ever pre-election gathering to announce plans to give free meals to nursery school children.

She will also rule out increasing the basic rate of income tax for the next five years.

Mr Sillars, who will not be at the conference, had "ripped up his membership card" by campaigning for a rival party, one furious MSP said.

Another accused him of bolstering the chances of Unionist parties by diverting regional, or 'list,' votes away from the SNP.

The Herald understands Mr Sillars actions have been raised with officials at party HQ.

Sources have confirmed that campaigning for another party is in clear breach of party rules.

Possible disciplinary action includes expulsion from the party, one said.

Mr Sillars' video in support of RISE features on the group's Facebook page and has been viewed almost 4000 times within days of its release.

Although he does not explicitly call for others to follow his lead, he says: "I'm voting RISE on the list in the Holyrood elections."

In a pitch to SNP supporters, he adds: "They’re going to table a resolution asserting the right of the parliament to hold an independence referendum when that parliament decides the time is right.

"RISE are asking people to sign a nationwide petition, in order to give their resolution even greater force."

RISE, which stands for Respect, Independence, Socialism and Environmentalism, is an alliance of left wing groupings that came together after the independence referendum campaign.

Candidates include Colin Fox, the former Scottish Socialist Party MSP, leading trade unionist Cat Boyd, and Jean Urquhart, the Independent MSP who quit the SNP in 2012 over its support for NATO.

They argue the SNP is so far ahead in the polls that under Holyrood's hybrid proportional system a regional vote for the SNP would not increase its final tally of MSPs.

Faced with a double threat from RISE and the Scottish Green Party, which also wants to steal its second votes, the SNP has adopted the slogan "Both Votes SNP."

It will feature heavily at this weekend's conference, even appearing on delegates' security passes.

Mr Sillars has been a member of the SNP for more than 30 and served as deputy leader in the early 1990s.

He was married to the late Margo MacDonald, who was expelled from the SNP in 2003 when she stood successfully for Holyrood as an Independent candidate.

He has never been afraid to challenge the party leadership, criticising Alex Salmond's referendum strategy and, in recent weeks, campaigning for a Brexit vote in defiance of official SNP policy.

Sillars first declared his support for RISE in December, when he said a regional vote for the SNP would be "wasted".

In a speech to SNP members in Edinburgh a fortnight ago, which was filmed and uploaded to a pro-independence internet channel, he dismissed the 'Both Votes SNP' message.

He said he was "a heretic and a rebel" but acknowledged most SNP members would back the party with their constituency and list votes.

He said he wanted Cat Boyd and Colin Fox elected to Holyrood because they were true socialists.

But his decision to go a step further and take part in a campaign video for the alliance has infuriated some SNP MSPs.

One said there was no evidence that RISE had a realistic chance of winning seats and that diverting second votes from the SNP would only increase the Unionist parties' chances.

But in a sign the party hierarchy may seek to brush the problem under the carpet, rather than draw attention to his campaigning, one cabinet minister said: "You just have to shrug your shoulders.

"Jim has been telling people to do all sorts of things down the years.

Mr Sillars said: "I'm 78 years old. I've been in politics 55 years. I'm not disconcerted by anything.

"I'm doing no more with RISE than I did in voting for Margo, indeed campaigning for Margo on the second vote.

"They are a bit late to be complaining about me now."