Jeremy Corbyn has launched a strong defence of his controversial frontbench reshuffle, saying it has made Labour “stronger, more diverse and more coherent” as he insists he can lead the party to general election victory.

Writing in the Observer after a turbulent week, Corbyn says his election as leader in September reflected a “deep-seated desire for change and a new direction in our politics” which, unsurprisingly, some in the party are finding difficult to adjust to. In terms that may alarm some of his MPs, who fear he is trying to bypass the parliamentary party over issues such as the renewal of Trident, he makes it clear that he will shape new policies through the “democratic participation of our own hugely expanded party and supporters”.

In a call for loyalty, Corbyn says the Tories under David Cameron are engaged in a systematic attempt to undermine democracy and that this must be challenged by Labour. He argues that new rules on voter registration, attempts to cut the number of parliamentary seats and slash funding for Labour amount to an unprecedented attack on democratic rights and freedoms.

However, his attempts to impose unity will come under further strain on Sunday when an MP appointed to head a key policy review on tax credits and child poverty will resign on BBC TV, in protest at being branded by the leadership as part of a “rightwing clique”.

Facebook Twitter Pinterest Alison McGovern MP will quit her policy review role over John McDonnell’s attack on the pressure group Progress. Photograph: Christopher Thomond/The Guardian

After three frontbenchers quit in protest at Corbyn’s reshuffle, Alison McGovern, MP for Wirral South, will launch a bitter attack on John McDonnell, Corbyn’s close ally and shadow chancellor, who said last week that the pressure group Progress, which she chairs, is full of people with “hard-right” conservative views.

Her appearance on the BBC’s Sunday Politics will also fuel a row that has erupted between Labour and the BBC over its coverage of the reshuffle, and comes as tensions over Corbyn’s attempts to change party policy on Trident reach new heights.

On 8 January, Corbyn’s office lodged an official complaint with the BBC, alleging it had orchestrated controversy over the reshuffle by “stage managing” the resignation of frontbencher Stephen Doughty live on air on BBC’s Daily Politics two days earlier. The BBC strongly rejected the claims and denied Labour accusations that it had breached its duty of impartiality.

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Appearing on the Sunday edition of the programme , McGovern, a former shadow Treasury minister, will explain her decision to quit. She told the Observer that she could not allow Progress, an organisation made up entirely of Labour members, which she said promotes “progressive policies”, to be insulted by the leadership: “To say we are rightwing and conservative is not like saying we are wrong or even stupid. By saying that, McDonnell is saying he knows what we think and what is in our heart, what we have spent our political lives thinking and caring about.”

McGovern will tell the programme, which is anchored by Andrew Neil: “We’re all Labour members and believe in having a Labour government. That’s what we are, nothing more, nothing less. I have been backed into a corner and I have no choice now but to stand up and say ‘This is who we are’, and we should just get on with the business of getting a Labour government.”

In a sign of growing tensions over Trident policy, senior party sources say they have seen documents that show the leadership wants to give the national executive committee, on which Corbyn loyalists have a majority, the power to change policy on key issues in exceptional circumstances. This would mean bypassing the national policy forum, made up of delegates from constituency parties. Labour policy at the general election, confirmed at last year’s annual conference, was to back renewal of Trident, but Corbyn fought his leadership campaign on a pledge to abandon it.

Maria Eagle was removed as shadow defence secretary in the reshuffle because of her support for renewal of Trident, which Corbyn has always opposed.

Corbyn is said to want to push change through so the party has a clear unilateralist line before a key vote on Trident in the next few months. Opponents say they think he will put the issue to the NEC and conduct an email survey of members to try to win grassroots support before whipping Labour MPs to vote against renewal.

Meanwhile, Hilary Benn, who saw off attempts by Corbyn’s office to remove him as shadow foreign secretary, remains in favour of Trident renewal. A spokesman for Benn said: “Hilary was not asked to, and has not changed his position on, Trident [during and since the reshuffle].”