David Lammy has set out plans to foster a new “civic nationalism” based not on skin colour, religion or ethnicity but on shared values as he lays the ground for a possible pitch at the Labour leadership.

The Tottenham MP would become the first BAME leader of the Labour party, were he to enter and win the race to succeed Jeremy Corbyn that will begin in the new year.

Writing in the Observer, Lammy suggests he will make a decision on whether to run over Christmas, having assessed who best can counter what he calls the “populist, ethnic nationalism” of Boris Johnson.

“The alternative to Boris Johnson’s ethnic nationalism that Labour should offer is a civic nationalism,” Lammy says. “Rather than basing national pride on biological heritage, skin colour or religion, civic nationalism says that we can be united around shared values and institutions.

“To foster this, we need to construct new spaces and places in which the UK’s diverse peoples can engage with each other and belong.”

Lammy suggests a series of radical reforms, including a British bill of rights and a codified constitution, a new proportional voting system to encourage more compromise in politics, a universal basic income and a compulsory national civic service in which all young people would be compelled to take part.

Lammy, whose support is strong in London, says he regards Corbyn as a friend but is scathing about his failings, which he says were responsible for Labour’s worst general election defeat since 1935.

“It was not Jeremy’s righteous belief in economic justice that lost Labour this election,” he says. “It was his perceived worldview, failure of competence, and mind-boggling decision to abdicate leadership on the biggest issue of the day [Brexit].”

To date, only two candidates have announced they will stand for the leadership: the shadow foreign secretary, Emily Thornberry, and shadow Treasury minister Clive Lewis.

The shadow Brexit secretary, Keir Starmer, is preparing to announce his candidature in the new year, while Rebecca Long Bailey, Lisa Nandy, Jess Phillips and Yvette Cooper are also considering entering the race. Shadow education secretary Angela Rayner, touted as a possible leadership contender, is more likely to stand instead for the role of deputy leader.

Long Bailey has yet to make any contribution to the debate over the party’s future direction since the election, amid signs that the Corbyn wing of the party is at odds over whether to promote her as a “continuity” candidate who will take forward the radical economic ideas championed by Corbyn and John McDonnell, or as a more independent-minded figure who wants to move on from Corbynism.

On Saturday night a senior source close to McDonnell, who has long promoted Long Bailey as his protege, said the shadow chancellor had made a “conscious decision” not to play any role in the leadership contest. Supporters of other candidates believe the Long Bailey camp is struggling to unite amid the bitter fallout following the election result.

On Sunday, also writing in the Observer, Labour’s former deputy leader Roy Hattersley makes the startling suggestion that if Long Bailey is chosen then moderate Labour MPs should refuse to accept her. “Despite the obvious truth that Jeremy Corbyn must take the blame for the worst result in almost 100 years, Rebecca Long Bailey, his anointed successor, is the favourite to succeed him as party leader,” Hattersley says.

If she were to win, he says, “genuine democratic socialists in the parliamentary party” should strike back. “The elevation of Long Bailey would provide an early opportunity to demonstrate that they mean business. The cause would be best served by an outright refusal to accept the imposition of a leader who does not command their confidence. A formal protest with a recorded vote would be almost as effective. Emboldened, they must then insist that the shadow cabinet is, once again, elected – giving its members an independent authority which they would not possess as leader’s nominees.”