John Swinney has announced that he is to stand down as SNP leader.

Following a poor round of election results in the recent European poll, pressure had intensified on Swinney.

Addressing journalists on Tuesday, the SNP leader conceded that the party's prospects were being damaged by "constant speculation about my leadership".

"I couldn't allow a situation to develop where good electoral prospects for the SNP were obscured by continual speculation about John Swinney," he said.

He will continue in the leadership until the party conference in the autumn and has confirmed that he intends to remain as an MSP.

Undermined

And he warned his critics that they risk undermining the entire party unless they adopt greater discipline and loyalty to the new leadership.

"Given the scale of the challenges we face, what our members need and what I appeal for today is unity within the Scottish National Party to ensure our success," he said.

"No member of the SNP should ever under-estimate the damage that is caused to our movement by the loose and dangerous talk of the few.

"Today I pledge my support and my co-operation to my successor, who will be elected for the first time by one member, one vote."

Briefing

Behind the scenes briefing against Swinney escalated over the weekend, with newspaper polls showing he had lost the confidence of party activists.

The leadership speculation mounted when the SNP polled under 20 per cent in the European parliamentary elections.

Swinney was elected MP for North Tayside on May 1, 1997 - becoming MSP for the same constituency in 1999.

In 2000 he was elected by the nationalist party as its convener, also becoming opposition leader in the Scottish parliament.

Prior to his election to Westminster Swinney worked with Scottish Amicable as a business and economic development consultant.

Since his election Swinney has been criticised for failing to boost the party's profile and electoral standing.

Lacking the charisma of his predecessor Alex Salmond, Swinney had become the subject of leadership speculation and behind the scenes manoeuvring.

Last year he saw off a leadership challenge, although he failed to capitalise on Labour's electoral woes following the Iraq war.

Contest

Swinney's move paves the way for what may become a divisive leadership poll which could expose the ideological divisions within the nationalist movement.

Nominations close on July 16 - with the new leader to be elected on September 3.

Those tipped for the leadership include Nicola Sturgeon, Kenny MacAskill and deputy leader Roseanna Cunningham.

Cunningham, who was informed of Swinney's decision on Monday evening, said: "Independence has been the great cause in John's political life.

"I know that John will continue to do all in his power to work for that cause and for the benefit of the people of North Tayside, the constituency he represents so assiduously."

Salmond, who is said by some to harbour ambitions to return to the leadership, said: "John Swinney is a good man laid low by extraordinary negativity of much of the Scottish press and a small faction within the SNP who were never reconciled with his leadership.

"As a result his achievements as national convenor, such as winning nine first past the post seats in the Scottish elections last year, and the process of party reform were belittled, and every setback amplified.

"Scottish politics is a sadder place for the fact that someone of John Swinney's inherent decency is unable to sustain his position."

Opponents

Political opponents also paid tribute to the outgoing leader - although they warned that the new leadership could face the same problems.

First minister Jack McConnell said: "In the end, John Swinney had no choice but to resign, having increasingly lost the debate to Labour over the last three years.

"But a change at the top won't make much of a difference. The real problem with the nationalists is not their leadership but their politics and their policies."

Liberal Democrat leader Charles Kennedy said: "I feel sorry for John and express best personal wishes to him. However, the problem for the SNP and political nationalism in Scotland and Wales alike is that their relevance is hugely diminished by the onset of devolution.

"Whoever emerges as the new leader of the SNP will be very hard pushed to overcome this dilemma."