A Green party candidate has withdrawn in a second marginal seat targeted by Labour, prompting speculation that more Greens could step aside as part of an informal anti-Conservative alliance not sanctioned by the central party.

John Tyne, the Green candidate in Chingford and Woodford Green, the constituency held since 1992 by the former Conservative leader Iain Duncan Smith, was “tactically withdrawing” to help Labour take the seat, a statement on the local party’s website said.

It comes little more than a week after the Greens stood down their candidate in the West Yorkshire seat of Calder Valley, held by the Conservative Craig Whittaker with a majority of 609 over Labour. In the 2017 election, the Greens took 631 votes.

There will be increased pressure for more such local agreements after the Brexit party announced it would not field candidates in any seats won by the Conservatives in the last election in an informal Brexit-based alliance.

Duncan Smith formerly had a majority of more than 10,000 in his seat in north-east London but in 2017 this fell to 2,400. The Greens took 1,200 votes then.

Labour has been heavily targeting Chingford and Woodford Green, where the party’s candidate is Faiza Shaheen, an economist and activist who is from the area.

The local coordinator for the Greens, Andrew Johns, said in a statement that while Labour and the Greens still had differences he had decided to stand down after Shaheen agreed to work with his party against airport expansions and to curb carbon emissions.

“Iain Duncan Smith has demonstrated through his horrendous record on welfare that he does not care for people who are struggling to make ends meet,” Johns said. “He also disregards the seriousness of the climate emergency, having repeatedly voted in parliament against measures to reduce carbon emissions.

“Any decision that can lead to him being unseated is one that will mean we are a step closer to that greener and fairer world.”

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Any such deals are made on a purely local basis and there is no expectation that Labour will reciprocate.

The Greens are taking part in the so-called remain alliance, under which the Liberal Democrats and Plaid Cymru have agreed to stand down in each other’s favour in 60 seats across England and Wales. Labour has refused to take part in any electoral pacts.

While Labour’s centralised structure would stop local parties standing down in any seats, the Greens are hugely localised, and such decisions are taken by individual branches.

In a statement, the party strongly indicated it did not support the idea. It said: “This was a decision taken by the local party. The Green party places local democracy and local parties at the heart of its operations and in this election we will be standing in the vast majority of seats across the country and more than in 2017.

“The first weeks of this campaign have shown how it is the Green party who are putting the climate front and centre of this election.

“While Labour are still supporting airport expansion across the country, we are putting forward a transformative Green New Deal which will usher in the green transport revolution so clearly needed if we are to make the UK carbon-neutral by 2030.

“If Labour were serious in their concern for the environment they should reconsider their isolationist position on arrangements.”

Q&A What is ‘tactical voting’? Show Hide Under the first-past-the-post voting system, tactical voting is when you vote for a party that you would not normally support in order to stop another party from winning. For example, in a constituency where the result is usually tight between a party you dislike and a party you somewhat dislike, and the party you support usually comes a distant third and has no chance of winning, you might choose to lend your vote to the party you somewhat dislike. This avoids ‘“wasting” your vote on a party that cannot win the seat, and boosting the chances that the party you dislike most will lose.

A wider so-called progressive alliance pact in 2017, which included the Greens and Lib Dems as well as Labour and was intended to allow parties a free run in some seats, had little impact with the Greens in particular complaining about a lack of cooperation from Labour.