Israel’s settlements in the West Bank amount to a “wicked cocktail” of occupation and illegality, a system akin to apartheid South Africa that brings shame on the Israeli government, the former Tory minister Sir Alan Duncan will say on Tuesday.

In one of the strongest attacks on the government of Binyamin Netanyahu by a frontline UK politician, Duncan will criticise the Israeli government for its “reprehensible” behaviour in encouraging and supporting the creation of “illegal colonies”.

The former international development minister – who was given a desk in the foreign office as the prime minister’s envoy to Oman and Yemen after he left the government in the summer reshuffle – said supporters of settlements should be regarded as extremists.

Duncan will make “crystal clear” that he fully supports Israel’s right to exist peacefully within the original borders agreed at its foundation in 1948. “Israel is Israel. It is a country. It is a nation. It is a legitimate state and, since1948, is a full member of the international community, on a par with any other country. Nobody can or should challenge its right to exist.”

However, likening the occupation of parts of Hebron to apartheid South Africa, Duncan will say: “Occupation, annexation, illegality, negligence, complicity: this is a wicked cocktail which brings shame to the government of Israel. It would appear that on the West Bank of the Jordan the rule of international law has been shelved.”

Duncan will make his comments in a speech to the Royal United Service Institute in London after MPs voted to recognise Palestine as a state. They carried the motion by 274 to 12 – a majority of 262 – after the Conservative MP Sir Richard Ottaway, who described himself as a strong supporter of Israel, said the recent annexation of 950 acres on the West Bank had outraged him and made him look like a fool. The vote – which came after the Tories and the Lib Dems gave their backbench MPs a free vote and Labour imposed a one-line whip – will not change government policy.

Duncan, who discloses in his speech that he raised his concerns in a letter to David Cameron and William Hague as a minister last year, says the time has come for Britain to say in clear terms that Israeli settlements on land occupied by Israel after the six-day war in 1967 amount to an “act of theft”. He will say: “This illegal construction and habitation is theft, it is annexation, it is a land grab – it is any expression that accurately describes the encroachment which takes from someone else something that is not rightfully owned by the taker. As such it should be called what it is, and not by some euphemistic soft alternative.

“Settlements are illegal colonies built in someone else’s country. They are an act of theft, and what is more something which is both initiated and supported by the state of Israel.”

Duncan will highlight the “amazing city” of Hebron where Palestinian families have been forced out of the centre of the city, he will say, by settlers. He will say: “One should not use the word apartheid lightly, but as a description of Hebron it is both accurate and undeniable. In South Africa it meant pass cards, no free movement, forbidden areas, and first and second-class citizens. So it is in Hebron.”

The former minister will say that the Israeli government actively encourages settlement activity. “Rightful Palestinian citizens are reduced to having nothing while illegal Israeli colonisers get everything. Water, electricity, access, protection: illegal settlers get the lot, the Palestinians next to nothing.

“Settlement activity is not carried out by some minority group outside the orbit of the Israeli state. Settlement activity is systematically initiated, implemented and supported by the Israeli government, who authorise, implement and protect the relentless illegal expansion of the borders of Israel. This is reprehensible.

“In addition to being illegal, settlement activity is very often violent, nasty, and brutal. Not all, but many settlers are heavily armed and aggressive.”

Duncan will also say that anyone who supports settlements should be regarded as an extremist. “Settlement endorsement, meaning the denial that they are illegal and the support for their consequences, is a form of extremism which we should not tolerate. Be it tacit, or be it explicit, such an attitude is simply not acceptable.”

Successive British governments have repeatedly made clear to Israel that they regard Israeli settlements on lands captured during the 1967 six-day war as illegal. But Duncan believes that Britain and other friends of Israel need to spell out to Netanyahu’s government that by ignoring calls to halt expansion it is acting outside the bounds of democratic nations.

The former minister will use particularly harsh language to criticise Netanyahu for ignoring concerns raised by Barack Obama. The Peace Now group criticised Israeli plans to build homes in the Givat Hamatos area of East Jerusalem on the eve of a meeting between Obama and Netanyahu.

Duncan will say: “Apart from the calculated insult to President Obama, it is impossible to overstate the criminal intent and strategic importance of Israel’s settlement plan, announced a fortnight ago, for Givat Hamatos. It would finalise the severing of Bethlehem from Jerusalem; it is to be built on Palestinian lands owned by the villagers of Beit Safafa; and it would destroy any easy connection between Palestinian neighbourhoods in south Jerusalem and a future Palestinian state.”

• This article was amended on 14 October 2014. It originally stated: … Duncan will criticise Tel Aviv for its “reprehensible” behaviour in encouraging and supporting the creation of “illegal colonies”. It should have said: … Duncan will criticise the Israeli government. This has been corrected.

