The London Borough of Camden last December refused an application by the owners of Utopia Village in Primrose Hill which requested the transformation of 23 of the business units into luxury flats.
The prior approval application had been made under new rules introduced in May 2013 which allow offices to be converted into homes without the need for planning permission.
Earlier this year, the owners of the business premises appealed the council’s decision to refuse the prior approval application. The appeal has been recovered to be decided by Pickles.
According to the Planning Inspectorate, it is the only office-to-residential prior approval appeal to date to have been recovered for determination by the secretary of state.
The signatories of the letter ask Pickles to uphold Camden's decision to refuse the application, which they describe as a "test case ... with other cases all over England watching to see what he decides".
They say that 300 jobs are at risk in the "historic business enclave".
The letter has been signed by Portas, Dame Joan Bakewell, the chief executive of the British Fashion Council Caroline Rush, playwright Alan Bennett, journalist and author India Knight, Camden mayor Lazzaro Pietragnoli and Guy Levin, executive director of the Coalition for a Digital Economy.
The letter warns that the office-to-residential permitted development rights are "now threatening to destroy many vibrant, mixed use communities forever".
It states that, while the policy had been intended to turn under-utilised or derelict office space into homes to help alleviate Britain's housing shortage, "in practice the reality is quite the reverse".
"Up and down the country, and in particular in thriving mixed small-business/residential communities, long-standing and start-up businesses alike are being ejected from their premises by landlords eager to capitalise on more lucrative housing development opportunities coupled with very limited planning restrictions," the letter states.
It calls on Pickles to make the "right decision" on the Utopia Village recovered planning appeal.
"The economic and social impact of this opportunistic use of the new rules is something that you can, and must, take into account in your decision on Utopia Village: as has been argued in submissions put before you, it is part of the balancing excercise that must be read into the process," the letter says.
"We, along with other communities and businesses in the same position around the country, will be watching and hoping that you make the right decision."
In a statement, planning minister Brandon Lewis said: "While we do not comment on individual planning cases, the government’s change of use reforms are providing badly needed homes such as studios and one-bedroom flats for young people.
"This is especially true in London where there is a particularly acute need for more housing. Our reforms are helping promote brownfield regeneration, protect the green belt and increase housing supply at no cost to the taxpayer."