The foreign secretary also claimed that dozens of migrants who have been removed from the flight list after individual legal challenges with “be on the next flight”.
Leaders of the Church of England have said the permanent relocation of asylum seekers to central Africa was an “immoral policy” that “shames Britain”.
But Ms Truss said: “Our policy is completely legal, it’s completely moral”, adding: “Those people need to suggest an alternative policy that will be work.”
There are only seven people still set to board the first flight scheduled to leave on Tuesday night. “We are expecting to send the flight later today – I can’t say exactly how many will be on the flight,” Ms Truss told Sky News.
Asked about the dozens of people pulled from the flight after successful legal challenges, Ms Truss said they “will go on future flights”, adding: “If they’re not on this flight, they will be on the next flight.”
Meanwhile, Boris Johnson accused lawyers representing migrants of “abetting the work of criminal gangs” as he stepped up his attack on the legal profession.
Opening this morning’s cabinet meeting, the PM’s claimed to be working with “humanity and compassion” in tackling small boat crossings – adding: “This government really unlike many others in the way that we represent … that tradition of welcoming people.”
Up to 130 people were originally told they could be sent to Rwanda under the home secretary Priti Patel’s highly-controversial scheme.
But the Home Office said at the end of last week that only 31 people were due to leave on the first flight. Since then a flurry of legal challenges have seen the passenger list dwindle.
The Care4Calais group said several more names were taken off the flight list on Monday – leaving only seven people cleared for deportation scheduled for 9.30pm on Tuesday evening.
The government will not reveal the cost of the 200-capacity flight operated by Spanish company Privilege Style – but industry sources have estimated to The Independent that it could be up to £500,000.
Ms Truss said she “can’t put a figure on it”, and argued that the plan to reduce the cost of illegal immigration into Britain over time was “value for money”.
The foreign secretary also insisted that “significant” numbers of people will be on one-way flights to Rwanda for asylum seekers by the end of 2022. She would not predict what the numbers will be, but told BBC Radio 4’s Today programme: “They will be significant.”
Home Office sources insisted that the department would push ahead with the flight – which the government has already paid for – even if there is just one person booked on board.
But campaigners are increasingly optimistic that the government will be forced to cancel Tuesday night’s flight if all the remaining passengers can make legal challenges.
Senior Church of England bishops, including the Archbishops of Canterbury and York, have criticised the plan for lacking morality.
A letter to The Times, signed by the Most Rev Justin Welby and the Most Rev Stephen Cottrell, says: “Whether or not the first deportation flight leaves Britain today for Rwanda, this policy should shame us as a nation.”
On Monday Court of Appeal judges rejected a last-ditch attempt by campaigners to have the first flight blocked until a full hearing on the lawfulness of the “offshore processing” scheme can be heard in July.