A man has admitted starting a huge fire which wrecked a historic church.

Ryan Haggerty, 25, sparked the inferno at St Simon's in Glasgow's Partick last July 28, with an adjoining house where an elderly nun lived also badly damaged.

Soot-covered Haggerty was clocked nearby and told a group of people: "Don't grass me in."

A judge heard how it would take "several millions" of pounds to possibly restore the 19th century building, which is the third oldest Catholic church in the city.

Haggerty today pleaded guilty to a charge of wilful fire-raising as he appeared in the dock at the High Court in Glasgow.

It emerged he already had a previous conviction for the same crime.

He was remanded in custody pending sentencing next month.

Haggerty lived in homeless accommodation close to St Simon's at the time.

A string of 999 calls were made around 2.30am that morning after it was spotted the popular church - built in 1858 - was alight.

Prosecutor Eilidh Robertson said: "Ryan Haggerty deliberately set fire to St Simon's.

"Due to the almost complete destruction of the building, little can be ascertained precisely how it started and where the seat of the fire was."

READ MORE: Fire at St Simon's Church in Glasgow's Partick as nearby homes evacuated

However, investigators believed it was sparked in the main chapel area with no evidence of any accelerant being used.

Ms Robertson said it took no more than ten minutes from the blaze being ignited and it then engulfing the building.

She added: "The fire was well developed and visible from several miles away.

"It was obvious the church was not salvageable."

Sister Mary Ross, 79, lived in the presbytery next door and was awoken by the blaze.

She managed to escape and was helped by fire crews but collapsed at one stage on seeing the destruction caused.

Residents of nearby flats and student accommodation also had to be evacuated.

People who stayed with Haggerty at a homeless unit close to the scene were on the street as well.

They clocked him coming from the direction of the church.

Ms Robertson stated: "He had what looked like soot on his hands and face. 

"They were suspicious he had started the fire and challenged him.

"He denied it although later stated, ‘you all think I started that fire, don't you? Don't grass me in.’"

Haggerty was charged with the crime on August 17.

The court heard the fire has left St Simon's "almost completely destroyed" with only external walls in place.

Civil engineering work continues almost a year after the inferno.

Ms Robertson stated: "If it were to be restored to its former construction then the cost would be several millions."

The advocate depute said parish priest Canon Peter McBride described the impact of what happened as "terrible and heart-breaking".

The church also had close links with the local Polish community dating back to World War Two.

Sister Ross meantime stated it was an "act of good fortune" that she got out when she did that morning.

She added: "I think there would have been a very good chance I could have got smoke inhalation when the fire happened and I did not wake up... possibly worse."

Prosecutors said the fire was to the danger of the sister's life.

There was no reason given as to why Haggerty committed the crime.

However, his lawyer Euan Dow said Haggerty had a long-standing drug addiction at the time.

Lady Scott deferred sentencing for reports until August 10 in Stirling.