David Cameron believes disruption caused by today's public sector strikes is "wrong", his official spokesman said.
Asked whether the Prime Minister felt strikes in the public sector could ever be justified, the spokesman said: "Does the Prime Minister think that people in this country - parents, commuters, users of public services - should have their routines disrupted? Of course he doesn't think it is right."
Asked for the PM's assessment of the impact of today's strikes, the spokesman responded that Cameron would say "any disruption any user of public services experiences is wrong".
Please #takeextracare on your journey if you're travelling this evening, fire crews are on strike until 7pm
Education Secretary Michael Gove told ITV News there is "no excuse" for teachers going on strike, which has "caused disruption for hardworking parents".
Mr Gove said: "There's no justification for this action and teachers, I hope, will draw the appropriate lesson from today which is that it is their responsibility to put children first and to be in the classroom teaching, not on the picket line striking."
Public sector workers in the Isle of Wight have joined protestors across the Meridian region who are unhappy about their pay and working conditions.
Cabinet Office Minister Francis Maude said fewer than 20% of civil servants were on strike today compared with a third in the last big walkout in 2011.
Mr Maude said today's disruption was the fault of union leaders and urged public sector workers that the right to strike must be exercised "responsibly".
He pointed out that only one in five members of Unite and Unison had taken part in ballots leading to the strike, adding that low turnouts strengthened the case for reform.