Nigel Farage's Ukip is to target at least 20 parliamentary seats at the next general election, using his party's success in Thursday's council elections as the launch pad for an all-out assault on the House of Commons, party officials have revealed.

In a move that will further unnerve the Tories, Labour and the Liberal Democrats – all of which have suffered from the Ukip surge – senior party officials said the next move would be to identify specific, mainly marginal, seats, where it now has a strong base of councillors. It is imitating the tactics that established the Liberal Democrats as a strong parliamentary force in the 1990s.

The extent of Farage's ambitions came to light as Lib Dem leader Nick Clegg faced a serious backlash from party malcontents, including at least two parliamentary candidates and several prominent councillors, as activists gathered names on a petition demanding he be replaced immediately by a new leader.

Ukip sources said the first task after the Newark byelection on 5 June would be to ensure that Farage himself wins a Commons seat next May. Mainstream parties have publicly doubted Ukip's capacity to break into parliament, but Thursday's results have convinced the party that, with the right focus, it can deliver a Ukip caucus.

"We don't want Nigel to be the only one," said the source. "The key is to focus ruthlessly on 20-30 target seats. This is the way for a smaller party to crack the first past the post system in parliamentary elections, as the Lib Dems did."

While Ukip's projected national share of the vote fell, it increased its number of councillors by more than 150 – from just nine in 2010 – by focusing resources on target areas. It acknowledges that it may, at best, win only a handful of seats in parliament next May, but that will mean it has a base in Westminster for the first time.

Among the seats it is likely to target are a batch in Essex, plus Rotherham, Great Grimsby, Great Yarmouth, Portsmouth South, Eastleigh, Broxtowe, North Thanet and South Thanet. As the fallout from Thursday's elections continued, the spotlight turned more on to the Lib Dems and the Conservatives. Despite its own underwhelming performance at the local elections, Labour was buoyed as a new poll by Tory peer Lord Ashcroft showed that Ed Miliband's party was 12% ahead of the Tories in 26 key marginal battlegrounds.

Clegg was in the firing line after his party lost more than 300 council seats and ceded control of two key councils – Kingston-upon-Thames (to the Conservatives) and Portsmouth. Ahead of the European election results – which could see the Lib Dems wiped out in Brussels – organisers of a rebel petition said two MPs were expected to sign in the next few days, and that more could follow.

Ros Kayes, a councillor who is also standing for parliament for the Lib Dems in West Dorset, confirmed that she had signed: "On the doorstep, Nick has become a hate figure – he seems more concerned with his own position than about ordinary people. This isn't about getting the knives out. It's about mature, tactical thinking about the best way forward for the party in the next five years."

Kayes added that Vince Cable would be the ideal replacement leader to take the party to the election, after which a permanent leader could be democratically chosen. While Clegg has insisted he will not quit and Lib Dem rules make it highly unlikely that the coup could be successful – they need either a majority of MPs or 75 local associations to back it – the unrest is clearly damaging.

After criticism of Miliband from some senior party figures, the party's mood was transformed by the Ashcroft poll. It put Labour 12 points ahead across 26 battleground constituencies, on 41% to the Tories' 29%, with an average 6.5% swing from the Conservative party to Labour – enough to oust as many as 83 Tory MPs, and secure Labour a healthy Commons majority. Ukip was on 18%, with the Lib Dems on 8% in the survey of 1,000 voters in each of the areas.

George Osborne warned that those who had deserted the Tories for Ukip would have to "live with the consequences for years" if they failed to switch back to the Tories. Dismissing Tory calls for an electoral pact with Ukip, he said the choice would be clear at next year's election – between Miliband and David Cameron – and that Labour must not be allowed to take power.

Speaking on BBC1's Sunday Politics, Boris Johnson, the mayor of London and a potential replacement for Cameron if the Tories lose the election, put pressure on the prime minister to push for EU immigration to be limited to skilled migrants who have a specific job to come to. "It's been my view for a long time that this should be on the table for the renegotiation," Johnson said.