Gordon Brown is facing a fork in the road over how he deals with immigration and party reforms, according to the man he fears is emerging as a new leader of the Left.

Jon Cruddas has irritated No 10 by turning down two job offers after his “punt from left field” brought him much attention in the party’s deputy leadership elections this summer.

The Dagenham MP, who won the first round of voting but lost out overall to Harriet Harman, is painfully aware of the suspicion caused by his refusal to enter Mr Brown’s “big tent”.

But he is frank about the possibility that Mr Brown’s party reforms will not deliver the democracy they promise and he is not afraid of setting the Prime Minister both timetables and tests. Most pressing – a “key litmus test” – is how Mr Brown deals with the fears created by demographic change, Mr Cruddas says in an interview with The Times.

Mr Cruddas is sharply critical of Liam Byrne, the Immigration Minister, whom he accuses of immature politicking and urges Mr Brown to engage instead in detailed “unsexy” policy work to deal with voters’ legitimate concerns. “There’s a fork in the road around this migration issue,” he says, praising the Liberal Democrats’ attempts to debate an amnesty for some illegal migrants.

“The Immigration Minister’s quote was, ‘We don’t want them getting to the head of the queue in terms of jobs and services’, which I thought was not a mature attempt to navigate through complex territory but an attempt to triangulate from the Right.”

He admits that the sight of Baroness Thatcher in Downing Street stuck in his craw but says that he accepts the need to “rebuild a wide and deep coalition that characterised new Labour at its best”.

He withholds judgment, however, on Mr Brown’s controversial party reforms that dilute union influence but promise to extend grassroots democracy.

“We will see,” he says. “They promise a more open, democratic party from the bottom up, built around participation and new forms of community involvement.

“If they don’t [deliver] then we return two years down the road,” he says. “It’s a proxy for Gordon Brown’s new politics. The debate in the party reflects a broader debate about a modern, pluralist democracy. If you can keep that model going in the party then that bodes well for him having a more listening approach towards policymaking in the country. I think the early signs are very good.”

He insists that he turned down Mr Brown’s job offers because his whole campaign for the deputy leadership was to contest the assumption that John Prescott’s empire was up for grabs. He too has heard the speculation that he is positioning himself to lead a left-wing faction. “I hear all this stuff about, ‘You are just anticipating problems down the road’ – well, I’m not, actually.

“My DNA is not factional or oppositional. My interest is in certain policy terrain and how you build modern political organisations. I’m not interested in oppositionalism, I am interested in changing the tenor of the Labour government and the nature of the party so as to allow for more durable Labour governments. I think it’s fair that we have a debate about some of the differences and all that.

“I’ve had a few debates,” Mr Cruddas pauses and then corrects himself, “discussions with Gordon Brown and they’ve been very open and supportive.”

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