The Rt Rev Richard Chartres told the congregation: “I hope you know that you’re in the thoughts and prayers of millions of people here.”
The Grenadiers will replace the Welsh Guards, who have suffered five fatalities during their tour in Helmand.
Commanding officer Lt-Col Roly Walker, 39, is among those being deployed. He was quoted by the Evening Standard as saying: “It will be demanding and testing but I’m really positive.”
His wife, Kate, said: “I am confident in what he does, I am very proud of him. We have a very good wives group called the SWAGs. We all look after each other.”
Prime Minister Gordon Brown suffered a setback in his efforts to bolster public support for the war when Labour MP Eric Joyce resigned as the ministerial aide to Defence Secretary Bob Ainsworth on Thursday.
In his resignation letter, former soldier Mr Joyce called on Mr Brown to set a five-year timetable for withdrawal of British troops and questioned public support for the conflict.
“I do not think the public will accept for much longer that our losses can be justified by simply referring to the risk of greater terrorism on our streets,” he wrote.
“Nor do I think we can continue with the present level of uncertainty about the future of our deployment in Afghanistan.”
Mr Brown defended the war in Afghanistan in a speech to the International Institute for Strategic Studies in London last night.
He said: “Each time I ask myself if we are doing the right thing by being in Afghanistan and if we can justify sending our young men and women to fight for this cause, my answer has always been yes. For when the security of our country is at stake we cannot walk away.”
Mr Brown is looking to speed up the exit of British troops from Afghanistan by training up more Afghan soldiers to take over in the fight against the Taliban.
Officials believe that an army of at least 200,000 Afghans would be needed before a full withdrawal of British troops. They anticipate that the Afghan army could be up and running by 2011.
