ITV production workers, journalists and other staff are staging a 24-hour strike in a dispute over pay, threatening programmes and planning a protest outside the TV giant's annual meeting.
Good Morning Britain was one of the first programmes affected, with journalist Martin Lewis announcing that he would not be presenting his regular Deals of the Week section.
Presenter Susanna Reid did not appear in the early part of the show but it is not clear whether she is taking part in the walkout.
Lewis, of moneysavingexpert.com, tweeted: "Sadly I won't be doing deals of the week on Good Morn Brit tomorrow due to ITV strike. As NUJ member I prefer not to cross picket lines."
Most of the journalists take part in the action work on ITV's regional news programmes. Most of ITV's national news is provided by ITN. According to ITV, 98 of its journalists are members of the National Union of Journalists and 52 of these voted in favour of a strike
Members of the technicians' union Bectu and the NUJ voted in favour of industrial action by 67 per cent and 77 per cent in protest at a 2 per cent pay award.
Unions have been pressing for a bigger wage increase in view of the TV company's profits.
A protest will be held outside ITV's annual meeting in London, including a Simon Cowell "lookalike".
Bectu general secretary Gerry Morrissey told the Press Association that staff were "very unhappy" with the pay offer.
"There is a huge disparity between the pay offered to staff and the bonus package to executives as well as shareholder dividends.
"Our members are helping to make the profits but are not sharing in them. The company's attitude to reward for its senior executives compared with the way it treats staff has opened up a huge gulf. ITV is profitable because of the efforts of its workforce not in spite of them. ITV staff make programmes and run the business day-to-day and yet they are not being rewarded fairly in basic pay.
"We have to break this destructive pattern where year on year senior executives' warm words patronise staff but it is only senior management who get the financial reward."
An ITV spokesman said: "We are fully prepared to maintain an open dialogue with union representatives following the ballot result, which saw 226 union members voting to strike out of a UK workforce of 3,000 employees.
"We have contingency plans in place to ensure that our programmes will continue to be broadcast and are confident that viewers will be unaffected by the proposed industrial action.
"We have made an above inflation one year pay increase of 2%, effective from January 1, which is on top of the 11.5% of pay rises over the last four years, some way ahead of other media sector pay awards.
"We are also the only UK broadcaster to pay the living wage. Eligible colleagues also received the maximum £1,200 bonus in their pay packets in March and we have increased the 2015 bonus opportunity to its maximum £1,500 which would be paid next year.
"ITV continues to make good progress but our focus on costs remains incredibly important across the business as we balance the need to continue to invest in growing the business, our people and the programmes that we create and broadcast."