The attorney general sacked by David Cameron over his dogged support for the European convention on human rights (ECHR) says he fears the prime minister will use next week’s party conference to dilute the UK’s commitment to the international treaty.

Dominic Grieve told the Guardian he believed any attempt by the prime minister to back away from the ECHR would be dangerous: “It’s incoherent, it’s a bit anarchic, it breaches our international legal obligations,” he said. “It’s a complete breach of precedent.”

Next Wednesday Cameron will give the leader’s speech to the Conservative party conference, where many people expect him to focus on the ECHR. It is party policy to scrap the Human Rights Act, which enshrines the ECHR’s principles in domestic law, should the Tories win an overall majority after the next election.

Grieve said he was particularly concerned by the suggestion “that we would only implement the ECHR if a parliamentary motion said that the government should” for each specific piece of legislation – a move that he said would leave Britain open to the claim that it was failing to adhere to its international obligations.

“I think there’s quite a risk this may be announced next week,” said Grieve, who had “been left with the impression that there may be things that are going to come up that I may be uncomfortable with.”

Two months ago Grieve was unceremoniously removed from office despite holding the widespread respect of the legal profession and MPs as attorney general.

His dismissal was deemed so unlikely that Grieve’s office had arranged media briefings for the following day. His removal – alongside those of Damian Green and Ken Clarke, two other ministers who firmly supported the ECHR – was widely seen as a deliberate purge of dissenting voices on any change in approach to the treaty, while opponents such as the home secretary, Theresa May, and justice secretary, Chris Grayling, saw their hands strengthened.

Although the vast majority of complaints brought against the UK at the European court of human rights fail, the convention has become a lightning rod for widespread dissatisfaction among Conservative supporters with perceived European meddling in British affairs – and the sense that human rights laws are excessive.

Despite the manner of his removal from office, Grieve said he would continue to support the prime minister. “I’m a good conservative,” he said. “I’m conservative to my fingertips.”

But, he added, in what may be seen as a veiled warning: “The principles of conservatism include upholding the rule of law and the United Kingdom’s international legal obligations. If the party of which I’m a member makes an announcement which has the potential to breach the law and those obligations then I will argue against it … It would be very unsatisfactory.”

Previously Grieve had only acknowledged such a rationale was “possible”. But he now views the accuracy of that analysis as “almost certain”. And he expressed his dismay that the prime minister had narrowed the range of views held by his senior team.

“All political parties, if they are to be successful, have to be broad churches,” he said. “If you start to narrow the focus of the government down to particular elements of the party, then it may leave you with short-term advantages in terms of presentation or not having to look at difficult issues, but I think in the long run you’re likely to be diminished.”

Grieve, who had already said he believed an attack on Islamic State forces in Iraq would be legal, reiterated his view that a hypothetical expansion of UK strikes in Syria could be justified as well. But he sounded a more cautious note on the wisdom of such a move, warning that “whether it is legal and whether it is desirable are not one and the same”.

Before backing action in Syria, “I’d want to see again what the government had to say,” Grieve said. “Syria’s an even more challenging area to operate in [than Iraq] because we have no partnership with the government, and that’s a serious, irredeemable part of the problem. If one is going to justify using military force in Syria one has to be very clear-sighted about what that force is in practice going to achieve.”