More than a quarter of children starting primary school are unable to communicate in full sentences as concerns grow about the amount of time they are spending in front of screens, the education secretary will say in his first major speech on social mobility.

Damian Hinds is expected to say on Tuesday that he wants to harness technology so parents can do more to help their children’s early language development.

The education secretary promised to halve within a decade the number of children lacking the required level of early speaking or reading skills. His department’s figures show that 28% of four- and five-year-olds do not meet communication and literacy levels expected by the end of the reception year.

“It is a persistent scandal that we have children starting school not able to communicate in full sentences, not able to read simple words,” Hinds will say. “This matters, because when you’re behind from the start you rarely catch up. Your peers don’t wait, the gap just widens. This has a huge impact on social mobility.”

Children with a poor vocabulary aged five are more than twice as likely to be unemployed at age 34 as children with good vocabulary, research shows.

Announcing an education summit this autumn to encourage parents to get involved in their children’s language development from an early age, Hinds will say: “The home learning environment can be, understandably, the last taboo in education policy – but we can’t afford to ignore it when it comes to social mobility.

“I don’t have any interest in lecturing parents here. I know it’s parents who bring up their children, who love them, who invest in them in so many ways, who want the best for their children. But that doesn’t mean extra support and advice can’t be helpful.”

The summit will bring together businesses, charities, tech companies and media groups to encourage more parents to read and learn new words with their children.

Hinds will say: “Whilst there are legitimate worries about screen time, media and modern technology can also help to raise awareness and build parents’ confidence around what they can do to help their child’s early language development.”

The announcement builds on £20m pledged to narrow the gap between disadvantaged children and their peers. However, Labour said the government figures showed that inequality across the education system was rising, with the most vulnerable children the worst affected.

The attainment gap between children in care and their peers in reading, writing and maths has been rising at key stage 1 since 2010. The proportion of care leavers who are not in education, employment or training has risen from 32% in 2010 to 40% last year.

The shadow education secretary, Angela Rayner, said: “The measure of our education system should be the support that it offers to the most vulnerable children, and the steps we take to level the playing field between them and their more affluent peers. Quite simply, the Conservatives are failing this test.”