Tony Blair is to be presented with claims that his decision to topple Saddam Hussein by force was illegal before his appearance at the Iraq inquiry this week.

Two senior Whitehall lawyers are expected to claim that the former prime minister's decision to send British troops to aid the US-led invasion was illegal as it did not have the clear backing of the United Nations.

Sir Michael Wood, the most senior lawyer at the Foreign Office before the war, will give evidence to the inquiry tomorrow. His deputy, Elizabeth Wilmshurst, who resigned in 2003 in protest over military action, will also appear. It is thought they will suggest they believed military force was illegal without an explicit UN resolution giving approval for the invasion.

Britain and the US tried to secure such a resolution to put an end to the legal doubts, but failed to convince their UN partners.

Ms Wilmshurst has never spoken publicly of events that led to her resignation; Sir Michael has never admitted advising the government that the war would be illegal.

Crucial evidence will also be given by Mr Blair's chief legal adviser, Lord Goldsmith, who will give evidence on Wednesday. The inquiry has already heard that the former attorney-general appeared to give his decisive legal backing to the invasion only days before it was launched, after a further meeting with Mr Blair. The inquiry team has also been shown documents from Lord Goldsmith suggesting he still had doubts about the legality of the invasion, even after a resolution putting further pressure on Saddam was secured. Lord Goldsmith eventually told the cabinet that resolution 1441, signed in November 2002, did form the legal basis for the invasion in March 2003.

Witnesses have placed the responsibility for deciding on the war's legality squarely on Lord Goldsmith. Geoff Hoon, the former defence secretary, said last week that it would not have been “appropriate” for the cabinet to debate Lord Goldsmith's conclusion.

“What he was saying was that this was lawful in his judgment, and I can't see how we could have had a sensible discussion going beyond that,” he said. A debate on legality demanded by Clare Short, the former international development secretary, was refused.

Sir John Chilcot and his inquiry team have been accused of being deferential to witnesses and unwilling to push them on embarrassing questions. Some bereaved relatives said Mr Blair's appearance will be a “waste of time” because the questioning will not be tough enough.

Samantha Roberts, whose husband Steve was killed in Iraq in 2003, said Mr Blair would probably “sail through unscathed”. She added: “He will be so well-briefed, so well-prepared, and I don't think there's any sort of chance he will encounter problems.”

Embarrassing associations with the government have arisen during evidence sessions, leading to claims the team is too wedded to the establishment to reach difficult conclusions.

Belfast Telegraph