Getting a broadcasting job is still about "who you know" and the industry favours well-off people from the south east of England, the director general of the BBC will say.
Tony Hall will declare the BBC has "one of the most diverse workforces in the UK" but will add that there is "no room for complacency" in a speech about diversity on Thursday.
The corporation boss will say he wants to get more women and more people from black, Asian and minority ethnic backgrounds into the most senior positions at the BBC.
The broadcasting sector is often "far from representative of the world around it", and "instead of being a force for social mobility, is too often a source of social exclusion", Lord Hall will say.
In his speech in Thurrock, he will say: "Now, I am passionate about giving everyone the chance to shine in the creative sector.
"It's why I founded Creative and Cultural Skills ... why I was so proud to open up the Royal Opera House to new and different talent ... why I am so determined to do the same at the BBC.
"But the truth is, broadcasting in particular remains a relationship-based, 'who you know' industry.
"Too often, employers offer placements and internships through networks or contacts.
"Of course, this marginalises those who don't have connections, especially those outside the big cities ... And it favours the well-connected and well-off from the south east of England.
"There are always plenty of highly-qualified young people happy to work for free. And the industry is always happy to take them.
"But that leaves even fewer opportunities for those with fewer means - and especially for those who can't afford to get to London."
Lord Hall will say: "Getting the very best at the BBC means making sure we draw on all of the country's diverse talent.
"Ours is already one of the most diverse workforces in the UK - as far as we know, we're more diverse than any other major broadcaster, more diverse than the civil service, more diverse than any FTSE 100 company that reports its figures.
"But the targets we have set ourselves to reach by 2020 are among the most ambitious and stretching of any organisation. For the BBC, it's right that that's the case."
He will add: "We know there's plenty more to do - and no room for complacency. So we're looking at how we can go further. For me, one of the real priorities is to get more women, and more people from black, Asian and minority ethnic backgrounds, into our most senior leadership positions."
The BBC said it has met its 2017 targets, with 14.5% of the workforce coming from black, Asian or minority ethnic backgrounds, including more than 10% of its leadership.
More than 48% of BBC staff are women, approaching the corporation's 50% target set for 2020, and more than 10% of the staff identify as lesbian, gay, bisexual or transgender.
The broadcaster said more than 10% of its staff say they have a disability - which it said is ahead of the 2020 target.