Up to 1.5 million children face being left behind in their speaking and understanding due to disruption caused by Covid, research suggests.
And a majority of teachers are worried that children who are behind will not be able to catch up.
The communication charity, I CAN says more help is needed for pupils starting secondary school.
The governments in England, Scotland and Wales are spending more than £3bn on plans to help pupils catch up.
For its report, Speaking Up for the Covid Generation, I CAN asked primary and secondary school teachers across England, Scotland, and Wales about the impact Covid 19 had had on their pupils.
It found slightly over two-thirds (67%) of primary school teachers believe the children they teach are behind with their speaking and/or understanding.
Almost two-thirds (60%) of secondary school teachers, who have pupils who are behind, were worried that these pupils would not be able to catch up.
I CAN estimates this means up to 1.5 million children risk being left behind and suggests these children will find it hard to cope in secondary school without more help.
The research suggests the two biggest reasons why children are struggling with speaking are not being able to talk face-to-face with their friends (70%), and the overuse of tablets/phones and computers (69%).
'Forgotten how to play with each other'
Sarah Murphy, head teacher of Northwood Community Primary School in Liverpool, says pupils coming back into school have "really delayed social skills due to the lockdown and a lack of interaction with their peers".
"When they came back, pupils had to sit in rows instead of tables of four, but that just exacerbated the problem to the point we've returned to collaboration tables," she says.
The school has a dedicated unit to help pupils with language and communication skills and has also been doing therapeutic work with children.
Ms Murphy says there have been a lot more issues in the playground.
"There's been little purposeful play. It's like the children have forgotten how to play with each other so we're having a lot more behaviour incidents at breaks and lunchtime that are spilling over into the school day so learning time is lost."
Carrie Hyland, deputy head for inclusion at the school, says pupils are having to relearn the basics.
"It's particularly prominent with the older pupils - they've almost forgotten how to be in a social group," she says.
"They're struggling now as much as they are with people they've known for a long time, so meeting new people and coming out of their shell - communication-wise and emotionally - is going to be a major challenge."
'They miss their friends'
Louise Hearn, a mum of three young boys, says her eldest son, Aiden, who is 10, has been very quiet and not really spoken to anyone during lockdown.
He communicates with friends on Zoom and playing online games.
She says Aiden's communication was not good prior to the pandemic but has become worse. Lockdown meant his language and speech therapy was put on hold.
Leon, who is eight, does well in school but became less communicative and withdrawn when he couldn't go to school, she says.
"Leon is a natural chatterbox but school-wise, he just seemed to close up. He didn't want to take part in the Zoom lessons so much in the end.
"Friends-wise, he did get upset because there was no break time for kids, He couldn't really talk to them. He did cry a couple of times, saying how much he wanted to go back to school, how he missed his friends."
Jane Harris, I CAN chief executive said: "For 1.5 million children to be struggling to be able to speak and to understand what is being said to them should be a wake-up call to government and the education sector.
"Our survey shows that teachers in the classroom are not able to support the children who need their help because the support the government is offering is only for four and five-year-olds."
Government spending plans include:
A Department for Education official said: "Our national curriculum for English reflects the importance of spoken language in pupils' development and we are investing in targeted support for those from disadvantaged backgrounds who may be at risk of falling further behind."