The Education Secretary, Alan Johnson, has had his candidacy for the deputy leadership of the Labour party endorsed by a string of ministers.

Many of the middle-ranking MPs had warm praise for the tenacious way Johnson stood up to the Prime Minister in defence of gay equality laws.

Last month Mr Blair and the Communities Secretary Ruth Kelly were at the centre of a cabinet row over anti-discrimination legislation.

They wanted to grant exemptions to the new Sexual Orientation Regulations to Roman Catholic-run adoption agencies.

The new rules, which outlaw discrimination against gay, lesbian and bisexual people when accessing goods and services, will come before Parliament this month.

Alan Johnson led the Cabinet’s opposition to the co-called Catholic tendency, phoning colleagues to canvass their views and vocalising the opinion of many Labour backbench MPs that equality legislation should never be amended to accommodate special interest groups.

Eventually the Prime Minister decided not to grant any exemptions.

A Downing St spokesman praised the Education Secretary for his defence of gay rights.

“In terms of the Cabinet, I think what the Prime Minister would say is he’d pay tribute to Ruth Kelly and people like Alan Johnson who highlighted the concerns on both sides of the argument.”

Seven ministers have now declared they back Mr Johnson to succeed John Prescott as deputy leader.

The PM and Deputy PM are expected to stand down at the same time, probably in June or July.

The ministers who have backed Mr Johnson represent Blairites such as Pensions Minister James Purnell and ministers from a range of departments across the government.

As well as his clear-minded defence of equality, Mr Johnson’s CV speaks to the Labour party’s heart.

He began his professional life as a postman and rose to become the leader of the Communication Workers Union.

He was a moderniser in that role, the only major union boss to support the redrafting of the totemic Clause 4 of the Labour party’s constitution, which aspired to re-nationalise all the “means of production.”

The decision to do so, taken by the party at a special conference in 1995, was a key symbolic moment in the birth of New Labour.

Bounced into a safe seat weeks before the 1997 general elecion, he became the first former union leader in 40 years to sit in Cabinet.

The support of Mr Purnell, who used to work as an adviser to Tony Blair, is seen as a key sign that Johnson’s bid for the number two job has really taken off.

Mr Purnell said: “Alan Johnson has emerged as one of the major players of this Labour Government.

“His commitment to equality, improving social mobility and reforming public services, combined with extensive experience inside and outside Westminster, make him the best person for the deputy job.”

When Mr Johnson was Trade and Industry Secretary in early 2006, he told the Prime Minister that there would be no exemptions from the Sexual Orientation Regulations.

Mr Blair then moved Mr Johnson in the May 2006 reshuffle, and at the same time transferred responsibility for the regulations from Trade and Industry to the new Department for Communities, and appointed Ruth Kelly as Secretary of State.

Unlike the race to succeed Tony Blair, the deputy’s job is not going to be a coronation and the field is still wide open.

There are four other declared candidates for the deputy role and Peter Hain has even more cause to claim the mantle of gay rights champion.

As Secretary of State for Northern Ireland, Hain pushed through the Sexual Orientation Regulations in the teeth of opposition from Unionist parties. While Ruth Kelly delayed the regulations in the rest of the UK, Northern Irish LGB people already benefit from their protection.

Hilary Benn is also a declared candidate but it seems unlikely that the International Development Secretary will survive in a race against Johnson and Hain.

Harriet Harman, a declared candidate, also spoke up for the regulations.

She entered the race because she thinks the Labour party “needs” a woman deputy leader.

Although not a Cabinet minister, Harman broadly represented the views of most Labour MPs when she said in relation to the proposed Catholic opt-out that one cannot be “a little bit against discrimination,” and her comments got widespread publicity.

Backbench MP Jon Cruddas is busy touring the country with a message of grassroots renewal, and could have strong appeal to disaffected party members.

The elections of a leader and deputy are one of the few real democratic processes left in the Labour party.

They will probably take place five weeks after Mr Blair announces his intention to go.

Any MP who wants to stand needs the backing of 12.5% of Labour MPs, at present that means 44 endorsements.

The party uses an electoral college to elect its leadership, split three ways between the MPs and MEPs, the party membership and members of affiliated unions. That means over a million people will get a vote. Any candidate that gets 50% or more of the vote wins outright, otherwise further elimination ballots will be held.

The influence of the MPs on the result makes it unlikely that Mr Cruddas will win, even if he gets every party member to vote for him.

Other Cabinet members are rumoured to want the job but given the strength of Hain and Johnson’s positions it seems unlikely they will end up running.

Jack Straw is tipped to be the next Chancellor, and in any case as Foreign Secretary during the start of the Gulf war he is tainted with that policy disaster.

Similarly Tessa Jowell, once the darling of the party, has been battered by storms surrounding her husband’s business dealings and the Olympics budget.

Hazel Blears might think she has a chance, but as party chairman she has the responsibility of overseeing the process.

In any case her ultra-Blairite tendency to be ruthlessly on-message may not appeal to union or constituency Labour party members.