Mr Sinnott became NUT leader in 2004
Steve Sinnott, general secretary of one the biggest teaching unions in England and Wales, has died suddenly aged 56.
The National Union of Teachers, led by Mr Sinnott since 2004, said his death on Saturday was "a tragic loss". He leaves a wife and two children.
PM Gordon Brown said the former teacher and unionist had been "inspirational".
Mr Sinnott's death at home from a suspected heart attack comes just days after the union voted for its first national strike for more than 20 years.
He was born in Liverpool and began his teaching career there at a comprehensive in Toxteth in 1975, continuing teaching until his election as NUT deputy general secretary in 1994.
Mr Sinnott leaves a wife, Mary, a son, daughter and two grandchildren.
His son - also called Steve Sinnott - paid tribute to a "wonderful" father.
He said in a statement: "Our shock at the sudden loss of the most wonderful son, husband, father, granddad and teacher is immeasurable.
"If we can take any comfort it is that Dad died at home with his inspiration, my mum Mary, at his side."
The prime minister said: "Steve Sinnott was inspirational in his devotion to teaching, not just for children in Britain, but around the world.
"His commitment to teachers and education will be remembered."
Ed Balls, secretary of state for children, schools and families, also paid tribute to the union leader, expressing admiration for his "commitment to the teaching profession".
The minister said he was "deeply shocked and saddened" at the news of the union leader's "untimely death".
"Steve was a passionate advocate for teachers and a great believer in the power of education to transform the life chances of children and young people," said Mr Balls.
"I know from our many meetings and phone calls that he believed strongly that every child matters and deserves the best.
"We shared a common goal to put the interests of all children and young people at the heart of our policies.
"Of course, we did not always see eye to eye on every issue. But we never had an angry word."
The minister said Mr Sinnott "will be sadly missed".
Brendan Barber, general secretary of the TUC, said: "Steve's death comes as a great shock.
"He was a friend and a colleague who was deeply committed to the best of trade unionism and the British education system.
"Teachers, parents and students have lost a doughty fighter."
The leader of the rival union NASUWT, Chris Keates, said Mr Sinnott's passing was "a great tragedy for his family, for his union and for the trade union movement".
Ms Keates added: "Whatever the differences between our two unions we have always been united in wanting to do the best for teachers."
The union leader's death comes just days after his union voted to strike on 24 April in a campaign to stop the government imposing real-terms pay "cuts" on teachers.
The one-day strike could close half the schools in England and Wales.
Distraught NUT colleagues described Mr Sinnott as "the most compassionate of men".
Following her colleague's death, NUT acting general secretary Christine Blower said: "I know that he would have wanted the union to go ahead with all its campaigns because he believed in all of them with his heart as well as his head.
"At a later stage we will have the opportunity properly to remember Steve Sinnott, to honour his achievements and to celebrate his life.
"For now, the best way to mark our respect would be to maximise the effect of the campaigns to which he was so committed."
Mr Sinnott had been a member of the NUT since 1974.