An imprisoned British-Iranian mother would face separation from her three-year-old British daughter under a “two-tier system” which means the UK government denies help to people with dual nationality, an MP has said.

Tulip Siddiq, who is Nazanin Zaghari-Ratcliffe’s local MP, said she had been shocked by a meeting with the Foreign Office in which she said a minister implied the government was willing to help the charity worker’s young daughter, Gabriella, who is solely a British citizen, more than her.

The UK government has raised Zaghari-Ratcliffe’s case with Iranian authorities since her arrest in Iran last year as she attempted to return to Britain with Gabriella after visiting family. Dual nationals, however, are not able to access the same levels of consular assistance.

Siddiq told the Guardian her meeting before the general election with Tobias Ellwood, who has since moved to the Ministry of Defence, had an “ultra-defensive atmosphere” and said she left feeling shocked at the response from the government.

“I had the strong impression from the meeting that their priority was making sure they got Gabriella back to the UK, seeing as she is a full British citizen, rather than helping Nazanin. That would take the pressure off them, given Nazanin is a dual citizen,” the Labour MP said.

Gabriella is being looked after by her Iranian grandparents while her mother is in prison. Zaghari-Ratcliffe, who travelled to Iran on her Iranian passport, is allowed some irregular access to her daughter, which her family have said is a lifeline for her while she is imprisoned.

The Foreign Office’s current guidance meant Britain “effectively operates a two-tier system for citizenship when protecting British citizens abroad”, Siddiq said.

The guidance states the UK “would not normally offer support or get involved in dealings between you and the authorities of that state” if a dual national is arrested in a country where they also hold citizenship.

A Foreign Office spokesman said the government may make an exception if it judged a person to be particularly vulnerable, for example, if the case involves murder, forced marriage or an offence which carries the death penalty.

“However, the help we can provide will depend on the circumstances, and the country of the person’s other nationality agreeing to it,” he said. “All UK passports contain a warning about dual nationality. In the notes section, it says that British nationals who are also nationals of another country cannot be protected by Her Majesty’s representatives against the authorities of that country.”

Siddiq said the Foreign Office must take a more robust approach to defending the rights of dual nationals abroad, with more than 600,000 Britons who hold additional passports not automatically entitled to consular protection.

“Our law must change to ensure greater protection when dual nationals are detained,” she said. “FCO staff care about Nazanin’s plight, but they are hamstrung by an approach that isn’t strong enough. The government must state that there should be no exception to taking clearly documented action on behalf of all UK nationals facing breaches of their human rights.”

The MP for Hampstead and Kilburn will lead a debate in Westminster Hall on the topic on Tuesday, and has also drafted a 10-minute rule bill which would give Foreign Office officials the power to take a more robust and transparent approach. If selected, the bill would see a policy of escalation formalised and families and legal representation of the detained made aware of the process.

Zaghari-Ratcliffe, 38, was arrested at Tehran airport in April 2016, accused of attempting to orchestrate a “soft overthrow” of the Islamic republic, charges her husband, Richard Ratcliffe, said were complete fabrication and linked to her work as a project manager for the Thomson Reuters Foundation.

Her conviction was upheld by Iran’s supreme court in April this year. Ratcliffe, has said he was deeply concerned for his wife’s physical and mental health while she was being held in Tehran’s Evin prison.

In June, the foreign minister Alistair Burt acknowledged that dual citizenship made it far tougher for the UK to act. “The Iranian position on dual nationality makes progress difficult and we do not interfere with the legal systems of other countries,” he said.

His predecessor said there were similar difficulties. “The Iranian government does not recognise dual nationality and does not permit our consular staff to visit British-Iranian dual nationals detained there,” Ellwood told MPs in September last year.

Siddiq said she was frustrated the government had never openly called for Zaghari-Ratcliffe’s release or publicly declared her innocence.

“She is held on ridiculous charges of espionage that the government has not refuted despite numerous requests,” the MP said. “I do think there is more than meets the eye behind the lack of action.”

Dozens of dual nationals are believed to be imprisoned in Iran with little recourse to consular support. One lawyer told the Guardian in April as many as 40 could be imprisoned in the country, among them Kamal Foroughi, a British-Iranian businessman, has been in jail since 2011.

His MP, the Conservative Oliver Dowden, has written a previous joint letter with Siddiq to the Foreign Office, signed by 216 fellow MPs.

Siddiq said she was hopeful that she would get the backing from a number of Conservative MPs who had supported the letter for a change in the law. “This isn’t about me opposing the Conservative government, this is about taking constructive action to protect the rights of vulnerable people,” she said.