An internal Liberal Democrat document reveals that the party is braced for a complete wipeout in the European parliamentary elections.

As voters go to the polls for the European elections across the UK and local elections in England and Northern Ireland, senior party figures have been briefed to say that a failure to win any seats in the European parliament should be "expected" at this stage in the electoral cycle for a governing party.

The document, the contents of which have been leaked to the Guardian, advises Lib Dem spokespeople about what to say if the party wins between no seats and two seats in Strasbourg.

In that scenario, the document advises party figures to say: "Disappointed with the result but the party remains resolute and this was expected at this point in the electoral cycle."

Amid rumblings about Nick Clegg's leadership, the document also says that the party's best hopes are to win five seats in Strasbourg – more than halving the 11 seats the party won in 2009.

A wipeout, or even a dramatic reduction, in the Lib Dem contingent in the European parliament would represent a serious blow to Clegg's authority. The deputy prime minister, who made his name as an MEP, may face calls to end the coalition with the Tories – or even to stand down.

The embarrassing leak comes as Britain's final national electoral test before next year's general election begins. David Cameron is braced for defeat at the hands of Nigel Farage in the European elections because Ukip has been placed first in a series of opinion polls in recent weeks.

In the final YouGov/Sun poll on the eve of polling day Ukip was on 27%, followed by Labour on 26% with the Tories third on 22%. The Greens were fourth on 10% with the Lib Dems in fifth place on 9%.

The prime minister is expecting the Conservatives to come third in the European elections behind Ukip and Labour on the grounds that they are always "tricky" for a governing party. Tories on the right, who had indicated that they might move against the prime minister after a bad defeat, are keeping a lower profile amid the economic recovery and as they prepare for a major showdown on Europe in the runup to a possible 2017 referendum.

David Davis, the former shadow home secretary, indicated that some on the right will be prepared to criticise the Tory leadership over their tactics towards Ukip. He told the Guido Fawkes website: "The only way to fight Ukip is to occupy their ground. Cameron has started to try and do that, but it's too late…They only react when they are panicking and then no one believes it."

The Conservatives, who are also braced for losses in the council elections, want to turn the focus on Labour which should, No 10 believes, be making gains of 500 seats – around the number identified by the psephologists Colin Rallings and Michael Thrasher. Labour, which says it struggled in the European elections even at the height of Tony Blair's popularity, dismisses the Tory projection and is hoping for gains of 150 in the local elections.

Labour, which believes that the local elections will provide a much better pointer to the general election, believes it will do well if it wins 25% in the European elections – increasing the number of Labour MEPs from 13 to 22. But the main focus of the Labour campaign has been on areas in the local elections which include its Westminster target seats. Labour hopes to make progress in Redbridge, Croydon and Cambridge.

Labour sources say the Tories are in retreat in large parts of the country. They believe the Tories may suffer setbacks in key Westminster electoral areas such as Basildon, Peterborough, Southend and Swindon.

The private thoughts of the Lib Dems have been disclosed by an internal document presented to the Wheelhouse group, the body chaired by the general election coordinator Paddy Ashdown. The document suggests language to be adopted under three scenarios – from winning no seats to winning five seats. The height of the party's ambition in the document is five MEPs – a loss of six seats on the eleven they won in 2009. The Lib Dem numbers grew by one when Edward McMillan-Scott joined the party in 2010.

If the party retains two to three MEPs, senior Lib Dems have been advised to say that it represents "a good result considering the circumstances", while if it secures three to five MEPs, the document suggests talking about "a very encouraging result … much better than almost everyone predicted".

Lord Ashdown, the former party leader and Clegg's original patron, is due to come out fighting on his behalf in a newspaper article at the weekend. "Putting Paddy Ashdown in charge of the general election campaign [as campaign coordinator] is probably one of the brightest things Nick Clegg has done – he will fight to protect him," one party source said.

But the knives are already sharpening for the leadership amid concerns that the Lib Dems could suffer major setbacks in the local elections. There are fears that the party may lose control of Kingston upon Thames, south-west London, which the Lib Dems have held since 2002 – five years after the parliamentary seat was captured by Ed Davey, the energy and climate secretary.There have been complaints that major resources are being poured into the elections in Burnley borough council simply to save the council seat of Gordon Birtwistle because he is also the local MP. The council has been controlled by Labour since 2012 which has 26 councillors compared with 14 for the Lib Dems and five for the Tories.

A close eye will be kept on the business secretary Vince Cable, whose close ally Lord Oakeshott of Seagrove Bay has publicly called for Clegg to stand down, and on the chief secretary to the treasury Danny Alexander. Eyebrows were raised recently when Alexander met Lib Dem councillors in Essex – no parliamentary stronghold but an area with plenty of party members who would vote in any leadership contest.

One party source said: "Nick Clegg is probably safe if Vince and Danny are the main people in the frame. Vince will not wield the knife – he just wants the job to fall into his lap. Danny as leaders would be bonkers – he is the continuity candidate."

A Lib Dem spokesperson said of the document: "It's no surprise that, like all parties, we prepare for all sorts of outcomes. We know it will be a tough night but we are proud of our campaign. Nick Clegg is the only leader that has stood up to Ukip's divisive politics and defended British jobs and Britain's place in Europe."

One party source said the party is realistic about its chances because all the polls have placed the Lib Dems below the 13.7% they secured in the last European elections in 2009. The source said there is a fine line between failing to win any seats and winning five because the special form of proportional representation used rewards a party's percentage share but then assess a performance in relation to other parties.

What they'll say

Zero to two MEPs "Disappointed with the result but the party remains resolute and this was expected at this point in an electoral cycle."

Two to three MEPs "A good result considering the circumstances."

Three to five MEPs "Very encouraging result and is much better than almost everyone predicted."