Nick Clegg stuck by his Commons walkout over Europe
Immigration is causing a problem for schools because many young children do not speak any English, Liberal Democrat leader Nick Clegg has warned.
He said head teachers of primary schools were finding it difficult to deal with reception classes where pupils speak many different languages.
A lack of funds to deal with this was causing "real tensions in the classroom and under performance", he said.
However, he told the BBC: "I believe immigration is good for this country."
Mr Clegg's comments come as the parties enter the final days of campaigning for Thursday's local and mayoral elections.
He said his concerns about the impact of immigration on education followed conversations he had had with head teachers at primary schools.
He said they found "it very difficult to work with reception classes where children don't speak any English at all and they speak lots and lots of different languages".
"I strongly believe that we can't create the cohesion we want as British citizens unless we have a common language. It's the glue that binds us together," he told the BBC.
While it was impossible to "turn the clock back" on immigration, more needs to be done to "plan for it", he said.
"Real resources and pressure" were needed to ensure all young people speak English, including in the home, he said.
In an earlier interview, Mr Clegg told the BBC he planned to be Lib Dem leader for the "long term" and is pleased with the party's performance in the polls.
On Sunday his deputy Vince Cable said the party was "not doing as well as we hoped we would be" in the polls.
But in the third of a series of BBC interviews with the main party leaders, in the run-up to the local elections on 1 May, Mr Clegg told BBC Radio 4's Today programme: "I am in fact extremely pleased with where we are in the polls.
"The polls yesterday we're at 20%, that's considerably higher than 13% just a few years ago.
"It's far, far higher than we've ever been at this point in the political cycle two or three years after a general election."
He praised Mr Cable's performance as the Lib Dems' Treasury spokesman, in which he said he was "far outstripping" the Conservative shadow chancellor George Osborne.
Asked if Mr Cable - who was praised for his performance as temporary Lib Dem leader during last year's leadership election - was "shadowing" Mr Clegg, he replied: "I don't think Vince and I think about it in those terms at all."
He added: "We recognise we have different skills, different weaknesses, different strengths. I'm in this for the long run."
When Mr Clegg became Lib Dem leader in December, he was the party's third leader within two years.
Charles Kennedy quit in January 2006, forced out by a frontbench rebellion after admitting a drink problem. Sir Menzies resigned in October, blaming an age-obsessed media.
Since becoming party leader, Mr Clegg has faced some criticism over his decision to lead a walkout of Lib Dem MPs in the Commons after their call for a debate on Britain's future membership of the EU was refused.
He also had to accept the resignation of three members of his top team, after ordering all MPs to abstain from a vote on a narrower referendum on the EU reform treaty.
Questions about his judgement were also raised after he was quizzed about his sexual history by GQ magazine, telling interviewer Piers Morgan he had slept with "no more than 30" women.
On the GQ interview he said "wisdom with hindsight is an easy thing" but he had made a "split second" response which had been "taken out of context, interpreted, over interpreted and so on".
Asked if the experience had "tempered" him at all, he added: "As anyone who knows me will know, I have always brought to my politics energy, authenticity ...a great deal of energy and enthusiasm, none of that has dimmed in the slightest."
On the Commons walkout, which some critics have criticised as a stunt, he said he had wanted a "serious debate" on the whole question of Britain's place in Europe but had been "thwarted" by Parliament.
"I made my frustration about that perfectly plain ... I think it was right for us at the time given the rules that Parliament prevented us from having the democratic debate."
He said some MPs had not taken the "party position" and had "peeled off" on the EU reform treaty - as had happened in the Labour and Conservative Parties.
Looking to the future, he said his "overriding ambition" to double or more than double the number of Lib Dem MPs over two elections.
He said people knew the Lib Dems stood for civil liberties, had a long-standing commitment to the environment and wanted to reinvent the way party politics was conducted.