Fresh fears over the future of famous Scottish regiments such as the Argyll and Sutherland Highlanders and the Black Watch will be fuelled by the Defence Secretary today.

Philip Hammond will reiterate the Coalition Government's message, in a speech to the Royal United Services Institute (RUSI) in London, that rationalisation of the Army means some units will "inevitably" be lost or merged.

Although he will echo Prime Minister David Cameron's recent assurances that the regimental system will not be scrapped, Mr Hammond will warn of difficult decisions as the force shrinks from 102,000 to 82,000 personnel.

His comments will increase fears that a "political" solution will be found and that, while the names of famous Scottish regiments will be saved, some battalions could still be lost.

There has been speculation the Argyll and Sutherland Highlanders could be merged with others, such as the Black Watch, or that names could be attached to a Territorial Army battalion.

Mr Cameron has spoken recently of the importance of keeping "the regimental structure that we have". Today Mr Hammond will tell RUSI members "there is no question of abandoning the regimental system" although he will add: "Some units inevitably will be lost or will merge."

He will also praise regionally focused recruiting in a signal that the axe could fall on Scottish regiments. Mr Hammond has warned that those with significant numbers from Commonwealth countries, including Scots battalions, could be affected.

The Defence Secretary will also announce new "permanent partnerships" between light infantry battalions on deployment and reserve battalions. The move comes as part of plans to beef up the role of reservists.

He will also say that more reservists and contractors will be used in logistics to boost resources at the front line.

On the regiments issue, Mr Hammond will say today: "We value the history and heritage because they deliver tangible military benefits in the modern British Army. So there is no question of abandoning the regimental system.

"But that does not mean we can avoid difficult decisions as the Army gets smaller. And, in making those decisions, the military voice must prevail, ensuring that the Army remains the capable and agile force envisaged in the [2010 defence review]."

He will add the restructuring will "rethink the way we deliver every aspect of military effect in order to maximise capability at the front line".

A cross-party campaign was launched to save the Scottish regiments after Mr Hammond suggested earlier this year that he was not worried about the potential loss of historic "cap badges" – regimental names such as the Black Watch.