By Jo Coburn
BBC News political correspondent
After fourteen years of silence, Lord Kinnock has stepped into the political fray to attack the government's plans to reform secondary schools in England.
Lord Kinnock gave a typically passionate performance
For the first time in his career the former Labour leader openly criticised Tony Blair, when he addressed a packed Commons committee room on Thursday night, at the launch of a pamphlet by the left leaning think-tank Compass.
The leaflet, co-authored by former Downing Street adviser Fiona Millar, rejects the argument that a high-quality education system can only be achieved by some degree of selection.
Lord Kinnock gave a characteristically passionate performance, arguing that giving schools more freedom to set their own admissions policies would be a "step backwards for the education system".
He went on to say that self governing trusts would damage other local schools.
His intervention has changed the whole tone of what was already highly controversial debate.
One Labour MP said it was hugely symbolic and "ramped up the pressure" on the government.
Lord Kinnock's comments will certainly galvanise many in the Labour party who are unhappy with the plans for reform.
The fact that he has chosen this moment - and this issue - to express his concerns is a clear demonstration of the strength of feeling about education.
His speech will be seen as a warning shot across Mr Blair's bows to stay close to his party rather than try and rely on the votes of Conservative MPs to push through the legislation.
Selection in schools is an issue that goes to the core of what the Labour party is about.
On Friday, Education Secretary Ruth Kelly played down Lord Kinnock's intervention - to criticise him would only fan the flames of opposition.
But the fact that Downing Street also refused to be drawn into a discussion on the former Labour leader is an indication of just how politically sensitive the situation could become.