The Liberal Democrat leader Tim Farron has ruled out any form of coalition with the Tories or Labour after the general election as he sets out a bold ambition to attract enough Remain voters to form the main opposition party in parliament.

In a dramatic shift of strategy for a party that entered coalition with the Conservatives in 2010 in the “national interest”, Farron said in an interview with the Observer that there will be “no deal, no deal with anybody” under any circumstances.

He insisted that both the Tories and Labour were intent on driving through a hard Brexit, which would include taking the UK out of the single market, and that his party had a duty to offer a distinct alternative, including a policy that would keep open a possibility of the UK staying in the EU.

“There is no way we can countenance any kind of arrangement or coalition with the Conservative party and likewise with the Labour party led by Jeremy Corbyn,” Farron said. “He [Corbyn] accepted hard Brexit, he voted for it. He enabled it. It has put us in the situation we are now in.”

The move is clearly designed to reassure Tories who voted Remain in the EU referendum last June that he would not team up with Corbyn, as well as Labour Remainers who are angry with Corbyn over his Brexit policy, but who might be scared of voting Lib Dem if they thought the party could enter another coalition with the Conservatives.

Farron said the change of approach was approved by former leader Nick Clegg, who was deputy prime minister in the Tory-led coalition.

The new approach is also designed to spike Tory claims that a vote for the Lib Dems could lead to a “coalition of chaos” after the election.

In her first speech of the campaign last week, Theresa May sought to portray the election as a choice between five years of stability and strong leadership under her party and a “coalition of chaos” involving Labour, the Liberal Democrats and the SNP.

Stating that the 48% who voted Remain deserve a strong opposition to represent them, Farron made a direct appeal to them: “If you want to prevent hard Brexit, want to prevent us leaving the single market, if you actually want to give the British people the final say on the terms of the as yet unknown deal, which of course would allow people to vote to remain should they wish, and if you want actually a decent, proper opposition party in this country then we have this wonderful opportunity of an incredibly clear message that nobody else has. Those people who were on the losing side need someone to speak for them.”

The announcement, five days after May shocked Westminster by calling a snap election on 8 June, came as a new Opinium/Observer poll shows the Tories have stretched their lead over Labour by 10 points in a week. The Lib Dems have enjoyed a surge while support for both Labour and Ukip has fallen.

The survey, conducted on Wednesday and Thursday, puts the Conservatives on 45% (up 7 points compared with the previous week) while Labour is down three points on 26%. If replicated on polling day, a lead of that size could hand the Tories a landslide win and a Commons majority of around 100 seats. The Lib Dems are up four points on 11% while backing for Ukip is down five points to 9%. Backing for the Lib Dems suggests its clear anti-Brexit strategy is appealing to hardcore Remain voters, and the Tories’ appear to be benefiting as Ukip falls.

Writing in the Observer, former Labour home secretary David Blunkett urges Labour supporters to rally behind the party and to go out to vote whatever their reservations about the direction in which it is being led by Corbyn.

“This election is not about Jeremy Corbyn or those around him, and it is not about Brexit,” he said. “The truth is that we are fighting to maintain a functioning democracy in which all the levers of power do not rest in the hands of those commanding wealth and privilege.

“That is why winning every single vote for Labour, and returning decent hardworking Labour MPs and aspirant candidates to Westminster, takes precedent over any differences we have and any doubts which prevail.”

Blunkett added: “We have an obligation to ensure that Labour candidates succeed, and to avoid the accusation after the election that somehow the modernisers and those disparaged as ‘Blairites’ were responsible for anything short of victory.”

Farron said his party could make strides in many seats where the Remain vote was strong, but where Lib Dems have not traditionally been contenders. An example was Manchester Gorton where internal party polling suggests the Lib Dems can pull off a spectacular win despite having secured just 4% of the vote in 2015, when Labour won a majority of more than 24,000. More than 60% of voters in Gorton backed Remain in the referendum.

On Saturday May played down suggestions that the Conservatives would put up taxes insisting they were “a lower tax party” and that Labour’s “natural instinct” was to raise tax. But the prime minister would not say whether she would keep her party’s 2015 manifesto pledge not to raise VAT, national insurance or income tax. The shadow chancellor, John McDonnell, accused the government of planning “a tax bombshell” while former Lib Dem business secretary Sir Vince Cable accused May of being at war with her chancellor, Philip Hammond, over tax.

“Hammond admitted that taxes would have to rise, no doubt due to Theresa May’s hard Brexit that could leave anything up to a £100bn Brexit black hole in the public finances,” Cable said. “Theresa May should come clean on how she intends to fill the Brexit black hole if she won’t increase taxes.”