It is understood that the redundancies are being split proportionately across all areas of the business, including advertising sales, programme commissioning, TV production and support staff.
However, a source close to the company said it does not plan to cut the programming budget for 2016 or 2017 as ITV wants to keep investing in content and plans to make efficiencies off-screen.
ITV signalled that it wanted to take action now because 2017 could be a difficult year.
The X Factor broadcaster has already said it expects ad revenues to be down around 1% in the first nine months of this year and there are signs the market is worsening. Media buyers have suggested October will be down 8%, while November is expected to be down 7%.
ITV confirmed the 120 job cuts out of its UK workforce of about 3,500.
An ITV spokesman said: "At a time of political and economic uncertainty in our key markets, it’s important that we are in the strongest possible position to continue to invest in our strategy and to meet any challenges and opportunities ahead, as we continue to grow a successful business.
"We have taken costs out across ITV in a managed and sensible way over the last six years and we must continue to keep a tight control on spending to ensure that we are operating as efficiently and effectively as possible whilst maximising our ability to invest in the high quality programming that drives ITV’s success."
Adam Crozier, the chief executive, told investors in July after the Brexit vote that he was planning £25m of "overhead savings" in 2017.
ITV employs about 6,000 globally. It is thought staff outside the UK, chiefly in programme-making arm ITV Studios, will be unaffected.
Daily Mail & General Trust, the publisher of the Daily Mail, said last month it will cut 400 jobs.